MMUSIC Working Group F. Andreasen
Internet-Draft Cisco Systems
Intended Status: Proposed Standard July 11, 2008
Expires: January 2009
SDP Capability Negotiationdraft-ietf-mmusic-sdp-capability-negotiation-09.txt
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Abstract
The Session Description Protocol (SDP) was intended for describing
multimedia sessions for the purposes of session announcement,
session invitation, and other forms of multimedia session
initiation. SDP was not intended to provide capability indication or
capability negotiation, however over the years, SDP has seen
widespread adoption and as a result it has been gradually extended
to provide limited support for these, notably in the form of the
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offer/answer model defined in RFC 3264. SDP does not define how to
negotiate one or more alternative transport protocols (e.g. RTP
profiles) or attributes. This makes it difficult to deploy new RTP
profiles such as secure RTP or RTP with RTCP-based feedback,
negotiate use of different security keying mechanisms, etc. It also
presents problems for some forms of media negotiation.
The purpose of this document is to address these shortcomings by
extending SDP with capability negotiation parameters and associated
offer/answer procedures to use those parameters in a backwards
compatible manner.
The document defines a general SDP Capability Negotiation framework.
It also specifies how to provide attributes and transport protocols
as capabilities and negotiate them using the framework. Extensions
for other types of capabilities (e.g. media types and media formats)
may be provided in other documents.
Table of Contents
1. Introduction...................................................32. Conventions used in this document..............................73. SDP Capability Negotiation Solution............................73.1. SDP Capability Negotiation Model..........................73.2. Solution Overview........................................103.3. Version and Extension Indication Attributes..............14
3.3.1. Supported Capability Negotiation Extensions Attribute14
3.3.2. Required Capability Negotiation Extensions Attribute15
3.4. Capability Attributes....................................173.4.1. Attribute Capability Attribute......................173.4.2. Transport Protocol Capability Attribute.............193.4.3. Extension Capability Attributes.....................213.5. Configuration Attributes.................................213.5.1. Potential Configuration Attribute...................213.5.2. Actual Configuration Attribute......................293.6. Offer/Answer Model Extensions............................313.6.1. Generating the Initial Offer........................313.6.2. Generating the Answer...............................343.6.2.1. Example Views of Potential Configurations......403.6.3. Offerer Processing of the Answer....................423.6.4. Modifying the Session...............................433.7. Interactions with ICE....................................443.8. Interactions with SIP Option Tags........................453.9. Processing Media before Answer...........................463.10. Indicating Bandwidth Usage..............................47Andreasen Expires January 11, 2009 [Page 2]

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profile information, e.g. RTP/AVP or RTP/SAVP), media formats (e.g.
codecs), and various other session and media stream parameters that
define the session.
Simply providing media stream descriptions is sufficient for session
announcements for a broadcast application, where the media stream
parameters are fixed for all participants. When a participant wants
to join the session, he obtains the session announcement and uses
the media descriptions provided, e.g., joins a multicast group and
receives media packets in the encoding format specified. If the
media stream description is not supported by the participant, he is
unable to receive the media.
Such restrictions are not generally acceptable to multimedia session
invitations, where two or more entities attempt to establish a media
session, that uses a set of media stream parameters acceptable to
all participants. First of all, each entity must inform the other of
its receive address, and secondly, the entities need to agree on the
media stream parameters to use for the session, e.g. transport
protocols and codecs. To solve this, RFC 3264 [RFC3264] defined the
offer/answer model, whereby an offerer constructs an offer SDP that
lists the media streams, codecs, and other SDP parameters that the
offerer is willing to use. This offer SDP is sent to the answerer,
which chooses from among the media streams, codecs and other SDP
parameters provided, and generates an answer SDP with his
parameters, based on that choice. The answer SDP is sent back to the
offerer thereby completing the session negotiation and enabling the
establishment of the negotiated media streams.
Taking a step back, we can make a distinction between the
capabilities supported by each participant, the way in which those
capabilities can be supported, and the parameters that can actually
be used for the session. More generally, we can say that we have the
following:
o A set of capabilities for the session and its associated media
stream components, supported by each side. The capability
indications by themselves do not imply a commitment to use the
capabilities in the session.
Capabilities can for example be that the "RTP/SAVP" profile is
supported, that the "PCMU" codec is supported, or that the
"crypto" attribute is supported with a particular value.
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o A set of potential configurations indicating which combinations
of those capabilities can be used for the session and its
associated media stream components. Potential configurations are
not ready for use. Instead, they provide an alternative that may
be used, subject to further negotiation.
A potential configuration can for example indicate that the
"PCMU" codec and the "RTP/SAVP" transport protocol are not only
supported (i.e. listed as capabilities), but they are offered for
potential use in the session.
o An actual configuration for the session and its associated media
stream components, that specifies which combinations of session
parameters and media stream components can be used currently and
with what parameters. Use of an actual configuration does not
require any further negotiation.
An actual configuration can for example be that the "PCMU" codec
and the "RTP/SAVP" transport protocol are offered for use
currently.
o A negotiation process that takes the set of actual and potential
configurations (combinations of capabilities) as input and
provides the negotiated actual configurations as output.
SDP by itself was designed to provide only one of these, namely
listing of the actual configurations, however over the years, use of
SDP has been extended beyond its original scope. Of particular
importance are the session negotiation semantics that were defined
by the offer/answer model in RFC 3264. In this model, both the offer
and the answer contain actual configurations; separate capabilities
and potential configurations are not supported.
Other relevant extensions have been defined as well. RFC 3407
[RFC3407] defined simple capability declarations, which extends SDP
with a simple and limited set of capability descriptions. Grouping
of media lines, which defines how media lines in SDP can have other
semantics than the traditional "simultaneous media streams"
semantics, was defined in RFC 3388 [RFC3388], etc.
Each of these extensions was designed to solve a specific limitation
of SDP. Since SDP had already been stretched beyond its original
intent, a more comprehensive capability declaration and negotiation
process was intentionally not defined. Instead, work on a "next
generation" of a protocol to provide session description and
capability negotiation was initiated [SDPng]. SDPng defined a
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comprehensive capability negotiation framework and protocol that was
not bound by existing SDP constraints. SDPng was not designed to be
backwards compatible with existing SDP and hence required both sides
to support it, with a graceful fallback to legacy operation when
needed. This, combined with lack of ubiquitous multipart MIME
support in the protocols that would carry SDP or SDPng, made it
challenging to migrate towards SDPng. In practice, SDPng has not
gained traction and as of the time of publication of this document,
work on SDPng has stopped. Existing real-time multimedia
communication protocols such as SIP, RTSP, Megaco, and MGCP continue
to use SDP. However, SDP does not address an increasingly important
problem: the ability to negotiate one or more alternative transport
protocols (e.g., RTP profiles) and associated parameters (e.g. SDP
attributes). This makes it difficult to deploy new RTP profiles
such as secure RTP (SRTP) [RFC3711], RTP with RTCP-Based Feedback
[RFC4585], etc. The problem is exacerbated by the fact that RTP
profiles are defined independently. When a new profile is defined
and N other profiles already exist, there is a potential need for
defining N additional profiles, since profiles cannot be combined
automatically. For example, in order to support the plain and
secure RTP version of RTP with and without RTCP-based feedback, four
separate profiles (and hence profile definitions) are needed:
RTP/AVP [RFC3551], RTP/SAVP [RFC3711], RTP/AVPF [RFC4585], and
RTP/SAVPF [SAVPF]. In addition to the pressing profile negotiation
problem, other important real-life limitations have been found as
well. Keying material and other parameters for example need to be
negotiated with some of the transport protocols, but not others.
Similarly, some media formats and types of media streams need to
negotiate a variety of different parameters.
The purpose of this document is to define a mechanism that enables
SDP to provide limited support for indicating capabilities and their
associated potential configurations, and negotiate the use of those
potential configurations as actual configurations. It is not the
intent to provide a full-fledged capability indication and
negotiation mechanism along the lines of SDPng or ITU-T H.245.
Instead, the focus is on addressing a set of well-known real-life
limitations. More specifically, the solution provided in this
document provides a general SDP Capability Negotiation framework
that is backwards compatible with existing SDP. It also defines
specifically how to provide attributes and transport protocols as
capabilities and negotiate them using the framework. Extensions for
other types of capabilities (e.g. media types and formats) may be
provided in other documents.
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As mentioned above, SDP is used by several protocols, and hence the
mechanism should be usable by all of these. One particularly
important protocol for this problem is the Session Initiation
Protocol (SIP) [RFC3261]. SIP uses the offer/answer model [RFC3264]
(which is not specific to SIP) to negotiate sessions and hence the
mechanism defined here provides the offer/answer procedures to use
for the capability negotiation framework.
The rest of the document is structured as follows. In Section 3. we
present the SDP Capability Negotiation solution, which consists of
new SDP attributes and associated offer/answer procedures. In
Section 4. we provide examples illustrating its use and in Section5. we provide the security considerations.
2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. SDP Capability Negotiation Solution
In this section we first present the conceptual model behind the SDP
capability negotiation framework followed by an overview of the SDP
Capability Negotiation solution. We then define new SDP attributes
for the solution and provide its associated updated offer/answer
procedures.
3.1. SDP Capability Negotiation Model
Our model uses the concepts of
o Capabilities
o Potential Configurations
o Actual Configurations
o Negotiation Process
as defined in Section 1. Conceptually, we want to offer not just the
actual configuration SDP (which is done with the offer/answer model
defined in [RFC3264]), but the actual configuration SDP as well as
one or more alternative SDPs, i.e. potential configurations. The
answerer must choose either the actual configuration, or one of the
potential configurations, and generate an answer SDP based on that.
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capabilities and potential configurations. A more accurate depiction
of the actual offer SDP is therefore as follows:
+--------------------+
| SDP o1 |
| (actual |
| config |
| |
| +-------------+ |
| | capability 1| |
| | capability 2| |
| | ... | |
| +-------------+ | Offer
| | ----->
| +-------------+ |
| | potential | |
| | config 1 | |
| | potential | |
| | config 2 | |
| | ... | |
| +-------------+ |
| |
+--------------------+
The above structure is used for two reasons:
o Backwards compatibility: As noted above, support for multipart
MIME is not ubiquitous. By encoding both capabilities and
potential configurations in SDP attributes, we can represent
everything in a single SDP thereby avoiding any multipart MIME
support issues. Furthermore, since unknown SDP attributes are
ignored by the SDP recipient, we ensure that entities that do not
support the framework simply perform the regular RFC 3264
offer/answer procedures. This provides us with seamless backwards
compatibility.
o Message size efficiency: When we have multiple media streams,
each of which may potentially use two or more different transport
protocols with a variety of different associated parameters, the
number of potential configurations can be large. If each possible
alternative is represented as a complete SDP in an offer, we can
easily end up with large messages. By providing a more compact
encoding, we get more efficient message sizes.
In the next section, we describe the exact structure and specific
SDP parameters used to represent this.
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The solution consists of the following:
o Two new SDP attributes to support extensions to the framework
itself as follows:
o A new attribute ("a=csup") that lists the supported base
(optionally) and any supported extension options to the
framework.
o A new attribute ("a=creq") that lists the extensions to the
framework that are required to be supported by the entity
receiving the SDP in order to do capability negotiation.
o Two new SDP attributes used to express capabilities as follows
(additional attributes can be defined as extensions):
o A new attribute ("a=acap") that defines how to list an
attribute name and its associated value (if any) as a
capability.
o A new attribute ("a=tcap") that defines how to list transport
protocols (e.g. "RTP/AVP") as capabilities.
o Two new SDP attributes to negotiate configurations as follows:
o A new attribute ("a=pcfg") that lists potential
configurations supported. This is done by reference to the
capabilities from the SDP in question. Extension capabilities
can be defined and referenced in the potential
configurations. Alternative potential configurations have an
explicit ordering associated with them. Also, potential
configurations are preferred over the actual configuration
included in the "m=" line and its associated parameters.
o A new attribute ("a=acfg") to be used in an answer SDP. The
attribute identifies a potential configuration from an offer
SDP which was used as an actual configuration to form the
answer SDP. Extension capabilities can be included as well.
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o Extensions to the offer/answer model that allow for capabilities
and potential configurations to be included in an offer.
Capabilities can be provided at the session level and the media
level. Potential configurations can be included at the media
level only, where they constitute alternative offers that may be
accepted by the answerer instead of the actual configuration(s)
included in the "m=" line(s) and associated parameters. The
answerer indicates which (if any) of the potential configurations
it used to form the answer by including the actual configuration
attribute ("a=acfg") in the answer. Capabilities may be included
in answers as well, where they can aid in guiding a subsequent
new offer.
The mechanism is illustrated by the offer/answer exchange below,
where Alice sends an offer to Bob:
Alice Bob
| (1) Offer (SRTP and RTP) |
|--------------------------------->|
| |
| (2) Answer (SRTP) |
|<---------------------------------|
| |
| (3) Offer (SRTP) |
|--------------------------------->|
| |
| (4) Answer (SRTP) |
|<---------------------------------|
| |
Alice's offer includes RTP and SRTP as alternatives, where RTP is
the default (actual configuration), but SRTP is the preferred one
(potential configuration):
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
m=audio 53456 RTP/AVP 0 18
a=tcap:1 RTP/SAVP
a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_80
inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
a=pcfg:1 t=1 a=1
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The "m=" line indicates that Alice is offering to use plain RTP with
PCMU or G.729. The capabilities are provided by the "a=tcap" and
"a=acap" attributes. The transport capability attribute ("a=tcap")
indicates that secure RTP under the AVP profile ("RTP/SAVP") is
supported with an associated transport capability handle of 1. The
"acap" attribute provides an attribute capability with a handle of
1. The attribute capability is a "crypto" attribute, which provides
the keying material for SRTP using SDP security descriptions
[RFC4568]. The "a=pcfg" attribute provides the potential
configuration included in the offer by reference to the capability
parameters. One alternative is provided; it has a configuration
number of 1 and it consists of transport protocol capability 1
(i.e., the RTP/SAVP profile - secure RTP), and the attribute
capability 1 (i.e., the crypto attribute provided). Potential
configurations are preferred over the actual configuration included
in the offer SDP, and hence Alice is expressing a preference for
using secure RTP.
Bob receives the SDP offer from Alice. Bob supports SRTP and the SDP
Capability Negotiation framework, and hence he accepts the
(preferred) potential configuration for Secure RTP provided by Alice
and generates the following answer SDP:
v=0
o=- 24351 621814 IN IP4 192.0.2.2
s=
c=IN IP4 192.0.2.2
t=0 0
m=audio 54568 RTP/SAVP 0 18
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:4
a=acfg:1 t=1 a=1
Bob includes the "a=acfg" attribute in the answer to inform Alice
that he based his answer on an offer using potential configuration 1
with transport protocol capability 1 and attribute capability 1 from
the offer SDP (i.e., the RTP/SAVP profile using the keying material
provided). Bob also includes his keying material in a "crypto"
attribute. If Bob supported one or more extensions to the capability
negotiation framework, he would have included option tags for those
in the answer as well (in an "a=csup" attribute).
When Alice receives Bob's answer, session negotiation has completed,
however Alice nevertheless generates a new offer using the
negotiated configuration as the actual configuration. This is done
purely to assist any intermediaries that may reside between Alice
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and Bob but do not support the SDP Capability Negotiation framework,
and hence may not understand the negotiation that just took place.
Alice's updated offer includes only SRTP, and it is not using the
SDP Capability Negotiation framework (Alice could have included the
capabilities as well is she wanted to):
v=0
o=- 25678 753850 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
m=audio 53456 RTP/SAVP 0 18
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
The "m=" line now indicates that Alice is offering to use secure RTP
with PCMU or G.729. The "crypto" attribute, which provides the SRTP
keying material, is included with the same value again.
Bob receives the SDP offer from Alice, which he accepts, and then
generates an answer to Alice:
v=0
o=- 24351 621815 IN IP4 192.0.2.2
s=
c=IN IP4 192.0.2.2
t=0 0
m=audio 54568 RTP/SAVP 0 18
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:4
Bob includes the same crypto attribute as before, and the session
proceeds without change. Although Bob did not include any
capabilities in his answer, he could have done so if he wanted to.
Note that in this particular example, the answerer supported the
capability negotiation extensions defined here. Had he not, he would
simply have ignored the new attributes and accepted the (actual
configuration) offer to use normal RTP. In that case, the following
answer would have been generated instead:
v=0
o=- 24351 621814 IN IP4 192.0.2.2
s=
c=IN IP4 192.0.2.2
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t=0 0
m=audio 54568 RTP/AVP 0 18
3.3. Version and Extension Indication Attributes
In this section, we present the new attributes associated with
indicating the SDP Capability Negotiation extensions supported and
required.
3.3.1. Supported Capability Negotiation Extensions Attribute
The SDP Capability Negotiation solution allows for capability
negotiation extensions to be defined. Associated with each such
extension is an option tag that identifies the extension in
question. Option-tags MUST be registered with IANA per the
procedures defined in Section 6.
The Supported Capability Negotiation Extensions attribute ("a=csup")
contains a comma-separated list of option tags identifying the SDP
Capability Negotiation extensions supported by the entity that
generated the SDP. The attribute can be provided at the session-
level and the media-level, and it is defined as follows:
a=csup: <option-tag-list>
RFC 4566, Section 9, provides the ABNF [RFC5234] for SDP attributes.
The "csup" attribute adheres to the RFC 4566 "attribute" production,
with an att-value defined as follows:
att-value = option-tag-list
option-tag-list = option-tag *("," option-tag)
option-tag = token ; defined in [RFC4566]
A special base option tag with a value of "cap-v0" is defined for
the basic SDP Capability Negotiation framework defined in this
document. Entities can use this option tag with the "a=csup"
attribute to indicate support for the SDP Capability Negotiation
framework specified in this document.
The following examples illustrate use of the "a=csup" attribute with
the "cap-v0" option tag and two hypothetical option tags, "foo" and
"bar" (note the lack of white space):
a=csup:cap-v0
a=csup:foo
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a=csup:bar
a=csup:cap-v0,foo,bar
The "a=csup" attribute can be provided at the session and the media-
level. When provided at the session-level, it applies to the entire
SDP. When provided at the media-level, it applies to the media
description in question only (option-tags provided at the session
level apply as well). There MUST NOT be more than one "a=csup"
attribute at the session-level and one at the media-level (one per
media description in the latter case).
Whenever an entity that supports one or more extensions to the SDP
Capability Negotiation framework generates an SDP, it SHOULD include
the "a=csup" attribute with the option tags for the extensions it
supports at the session and/or media-level, unless those option tags
are already provided in one or more "a=creq" attribute (see Section3.3.2. ) at the relevant levels. Inclusion of the base option tag is
OPTIONAL; support for the base framework can be inferred from
presence of the "a=pcfg" attribute defined in Section 3.5.1.
Use of the base option tag may still be useful in some scenarios,
e.g. when using SIP OPTIONS [RFC3261] or generating an answer to
an offer that did not use the SDP Capability Negotiation
framework.
3.3.2. Required Capability Negotiation Extensions Attribute
The Required Capability Negotiation Extensions attribute ("a=creq")
contains a comma-separated list of option tags (see Section 3.3.1. )
specifying the SDP Capability Negotiation extensions that MUST be
supported by the entity receiving the SDP, in order for that entity
to properly process the SDP Capability Negotiation attributes and
associated procedures. There is no need to include the base option-
tag ("cap-v0") with the "creq" attribute, since any entity that
supports the "creq" attribute in the first place also supports the
base option-tag. Still, it is permissible to do so.
Such functionality may be important if a future version of the
capability negotiation framework were not backwards compatible.
The attribute can be provided at the session-level and the media-
level, and it is defined as follows:
a=creq: <option-tag-list>
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The "creq" attribute adheres to the RFC 4566 "attribute" production,
with an att-value defined as follows:
att-value = option-tag-list
The following examples illustrate use of the "a=creq" attribute with
the "cap-v0" base option tag and two hypothetical option tags, "foo"
and "bar" (note the lack of white space):
a=creq:cap-v0
a=creq:foo
a=creq:bar
a=creq:cap-v0,foo,bar
The "a=creq" attribute can be provided at the session and the media-
level. When provided at the session-level, it applies to the entire
SDP. When provided at the media-level, it applies to the media
description in question only (required option tags provided at the
session level apply as well). There MUST NOT be more than one
"a=creq" attribute at the session-level and one "a=creq" attribute
at the media-level (one per media description in the latter case).
When an entity generates an SDP and it requires the recipient of
that SDP to support one or more SDP Capability Negotiation
extensions (except for the base) at the session or media level in
order to properly process the SDP Capability Negotiation, the
"a=creq" attribute MUST be included with option-tags that identify
the required extensions at the session and/or media level. If
support for an extension is needed only in one or more specific
potential configurations, the potential configuration provides a way
to indicate that instead (see Section 3.5.1. ). Support for the
basic negotiation framework is implied by the presence of an
"a=pcfg" attribute (see Section 3.5.1. ) and hence it is not
required to include the "a=creq" attribute with the base option-tag
("cap-v0").
A recipient that receives an SDP and does not support one or more of
the required extensions listed in a "creq" attribute, MUST NOT
perform the SDP Capability Negotiation defined in this document. For
non-supported extensions provided at the session-level, this implies
that SDP Capability Negotiation MUST NOT be performed at all. For
non-supported extensions at the media-level, this implies that SDP
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Capability Negotiation MUST NOT be performed for the media stream in
question.
An entity that does not support the SDP Capability Negotiation
framework at all, will ignore these attributes (as well as the
other SDP Capability Negotiation attributes) and not perform any
SDP Capability Negotiation in the first place.
When an SDP recipient does not support one or more required SDP
Capability Negotiation extensions listed in the option tags, the
recipient MUST proceed as if the SDP Capability Negotiation
attributes were not included in the first place, i.e. all the
capability negotiation attributes should be ignored. In that case,
if the SDP recipient is an SDP answerer [RFC3264], the recipient
SHOULD include a "csup" attribute in the resulting SDP answer
listing the SDP Capability Negotiation extensions it actually
supports.
This ensures that introduction of the SDP Capability Negotiation
mechanism by itself does not lead to session failures.
3.4. Capability Attributes
In this section, we present the new attributes associated with
indicating the capabilities for use by the SDP Capability
Negotiation.
3.4.1. Attribute Capability Attribute
Attributes and their associated values can be expressed as
capabilities by use of a new attribute capability attribute
("a=acap"), which is defined as follows:
a=acap: <att-cap-num> <att-par>
where <att-cap-num> is an integer between 1 and 2^31-1 (both
included) used to number the attribute capability and <att-par> is
an attribute ("a=") in its "<attribute>" or <attribute>:<value>"
form, i.e., excluding the "a=" part (see [RFC4566]). The attribute
can be provided at the session-level and the media-level.
The "acap" attribute adheres to the RFC 4566 "attribute" production,
with an att-value defined as follows:
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att-value = att-cap-num 1*WSP att-par
att-cap-num = 1*DIGIT ;defined in [RFC5234]
att-par = attribute ;defined in RFC 4566
Note that white space is not permitted before the att-cap-num.
The "acap" attribute can be provided at the session level only when
the attribute capability contains session-level attributes, whereas
media level attributes can be provided in attribute capabilities at
either the media level or session-level. The base SDP Capability
Negotiation framework however only defines procedures for use of
media-level attribute capabilities at the media level (extensions
may define use at the session level).
Each occurrence of the "acap" attribute in the entire session
description MUST use a different value of <att-cap-num>.
Consecutive numbering of the <att-cap-num> values is not required.
There is a need to be able to reference both session-level and
media-level attributes in potential configurations at the media
level, and this provides for a simple solution to avoiding overlap
between the references (handles) to each attribute capability.
The <att-cap-num> values provided are independent of similar <cap-
num> values provided for other types of capabilities, i.e., they
form a separate name-space for attribute capabilities.
The following examples illustrate use of the "acap" attribute:
a=acap:1 ptime:20
a=acap:2 ptime:30
a=acap:3 key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyONQ6gAA
AAAGEEoo2pee4hp2UaDX8ZE22YwKAAAPZG9uYWxkQGR1Y2suY29tAQAAAAAAAQAk0
JKpgaVkDaawi9whVBtBt0KZ14ymNuu62+Nv3ozPLygwK/GbAV9iemnGUIZ19fWQUO
SrzKTAv9zV
a=acap:4 crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
The first two attribute capabilities provide attribute values for
the ptime attribute. The third provides SRTP parameters by using
MIKEY [RFC3830] with the key-mgmt attribute [RFC4567]. The fourth
provides SRTP parameters by use of security descriptions with the
crypto attribute [RFC4568]. Note that the line-wrapping and new-
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lines in example three and four are provided for formatting reasons
only - they are not permitted in actual SDP.
Readers familiar with RFC 3407 may notice the similarity between
the RFC 3407 "cpar" attribute and the above. There are however a
couple of important differences, notably that the "acap" attribute
contains a handle that enables referencing it and it furthermore
supports attributes only (the "cpar" attribute defined in RFC 3407
supports bandwidth information as well). The "acap" attribute also
is not automatically associated with any particular capabilities.
See Section 3.14. for the relationship to RFC 3407.
3.4.2. Transport Protocol Capability Attribute
Transport Protocols can be expressed as capabilities by use of a new
Transport Protocol Capability attribute ("a=tcap") defined as
follows:
a=tcap: <trpr-cap-num> <proto-list>
where <trpr-cap-num> is an integer between 1 and 2^31-1 (both
included) used to number the transport address capability for later
reference, and <proto-list> is one or more <proto>, separated by
white space, as defined in the SDP "m=" line. The attribute can be
provided at the session-level and the media-level.
The "tcap" attribute adheres to the RFC 4566 "attribute" production,
with an att-value defined as follows:
att-value = trpr-cap-num 1*WSP proto-list
trpr-cap-num = 1*DIGIT ;defined in [RFC5234]
proto-list = proto *(1*WSP proto) ; defined in RFC 4566
Note that white space is not permitted before the trpr-cap-num.
The "tcap" attribute can be provided at the session-level and the
media-level. There MUST NOT be more than one "a=tcap" attribute at
the session-level and one at the media-level (one per media
description in the latter case). Each occurrence of the "tcap"
attribute in the entire session description MUST use a different
value of <trpr-cap-num>. When multiple <proto> values are provided,
the first one is associated with the value <trpr-cap-num>, the
second one with the value one higher, etc. There MUST NOT be any
capability number overlap between different "tcap" attributes in the
entire SDP. The <trpr-cap-num> values provided are independent of
similar <cap-num> values provided for other capability attributes,
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i.e., they form a separate name-space for transport protocol
capabilities. Consecutive numbering of the <trpr-cap-num> values in
different "tcap" attributes is not required.
Below, we provide examples of the "a=tcap" attribute:
a=tcap:1 RTP/AVP
a=tcap:2 RTP/AVPF
a=tcap:3 RTP/SAVP RTP/SAVPF
The first one provides a capability for the "RTP/AVP" profile
defined in [RFC3551] and the second one provides a capability for
the RTP with RTCP-Based Feedback profile defined in [RFC4585]. The
third one provides capabilities for the "RTP/SAVP" (transport
capability number 3) and "RTP/SAVPF" profiles (transport protocol
capability number 4).
The ability to use a particular transport protocol is inherently
implied by including it in the "m=" line, regardless of whether it
is provided in a "tcap" attribute or not. However, if a potential
configuration needs to reference that transport protocol as a
capability, the transport protocol MUST be included explicitly in a
"tcap" attribute.
This may seem redundant (and indeed it is from the offerer's point
of view), however it is done to protect against intermediaries
(e.g. middle-boxes) that may modify "m=" lines while passing
unknown attributes through. If an implicit transport capability
were used instead (e.g. a reserved transport capability number
could be used to refer to the transport protocol in the "m="
line), and an intermediary were to modify the transport protocol
in the "m=" line (e.g. to translate between plain RTP and secure
RTP), then the potential configuration referencing that implicit
transport capability may no longer be correct. With explicit
capabilities, we avoid this pitfall; however, the potential
configuration preference (see Section 3.5.1. ) may not reflect
that of the intermediary (which some may view as a feature).
Note that a transport protocol capability may be provided,
irrespective of whether it is referenced in a potential
configuration or not (just like any other capability).
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The SDP Capability Negotiation framework allows for new types of
capabilities to be defined as extensions and used with the general
capability negotiation framework. The syntax and semantics of such
new capability attributes are not defined here, however in order to
be used with potential configurations, they SHOULD allow for a
numeric handle to be associated with each capability. This handle
can be used as a reference within the potential and actual
configuration attributes (see Section 3.5.1. and 3.5.2. ). The
definition of such extension capability attributes MUST also state
whether they can be applied at the session-level, media-level, or
both. Note that extensions can have option tags defined for them,
which can be registered with the IANA in accordance with the
procedures specified in Section 6.2.
3.5. Configuration Attributes3.5.1. Potential Configuration Attribute
Potential Configurations can be expressed by use of a new Potential
Configuration Attribute ("a=pcfg") defined as follows:
a=pcfg: <config-number> [<pot-cfg-list>]
where <config-number> is an integer between 1 and 2^31-1 (both
included). The attribute can be provided at the media-level only.
The "pcfg" attribute adheres to the RFC 4566 "attribute" production,
with an att-value defined as follows:
att-value = config-number [1*WSP pot-cfg-list]
config-number = 1*DIGIT ;defined in [RFC5234]
pot-cfg-list = pot-config *(1*WSP pot-config)
pot-config = attribute-config-list /
transport-protocol-config-list /
extension-config-list
The missing productions are defined below. Note that white space is
not permitted before the config-number.
The potential configuration attribute can be provided at the media-
level only and there can be multiple instances of it within a given
media description. The attribute includes a configuration number,
which is an integer between 1 and 2^31-1 (both included). The
configuration number MUST be unique within the media description
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(i.e. it has media level scope only). The configuration number also
indicates the relative preference of potential configurations; lower
numbers are preferred over higher numbers. Consecutive numbering of
the configuration numbers in different "pcfg" attributes in a media
description is not required.
A potential configuration list is normally provided after the
configuration number. When the potential configuration list is
omitted, the potential configuration equals the actual
configuration. The potential configuration list contains one or more
of attribute, transport and extension configuration lists. A
potential configuration may for example include attribute
capabilities and transport capabilities, transport capabilities
only, or some other combination of capabilities.
The configuration lists generally reference one or more capabilities
(extension configuration lists MAY use a different format). Those
capabilities are (conceptually) used to construct a new internal
version of the SDP by use of purely syntactic add and (possibly)
delete operations on the original SDP (actual configuration). This
provides an alternative potential configuration SDP that can be used
by conventional SDP and offer/answer procedures if selected.
This document defines attribute configuration lists and transport
protocol configuration lists. Each of these MUST NOT be present
more than once in a particular potential configuration attribute.
Extension configuration lists can be included as well. There can be
more than one extension configuration list, however each particular
extension MUST NOT be present more than once in a given "a=pcfg"
attribute. Together, the various configuration lists define a
potential configuration.
There can be multiple potential configurations in a media
description. Each of these indicates not only a willingness, but in
fact a desire to use the potential configuration.
The example SDP below contains two potential configurations:
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Note that white space is not permitted within this production.
Each attribute configuration list can optionally begin with
instructions for how to handle attributes that are part of the
actual configuration SDP (i.e., the "a=" lines present in the
original SDP). By default, such attributes will remain as part of
the configuration in question. However, if delete-attributes
indicates "-m", then all attribute lines within the media
description in question will be deleted (i.e., all "a=" lines under
the "m=" line in question). If delete-attributes indicates "-s",
then all attribute lines at the session-level will be deleted (i.e.,
all "a=" lines before the first "m=" line). If delete-attributes
indicates "-ms", then all attribute lines within this media
description ("m=" line) and all attribute lines at the session-level
will be deleted.
The attribute capability list comes next. It contains one or more
alternative lists of attribute capabilities. The alternative
attribute capability lists are separated by a vertical bar ("|"),
and each list contains one or more attribute capabilities separated
by commas (","). The attribute capabilities are either mandatory or
optional. Mandatory attribute capabilities MUST be supported in
order to use the potential configuration, whereas optional attribute
capabilities MAY be supported in order to use the potential
configuration.
Within each attribute capability list, all the mandatory attribute
capabilities (if any) are listed first, and all the optional
attribute capabilities (if any) are listed last. The optional
attribute capabilities are contained within a pair of square
brackets ("[" and "]"). Each attribute capability is merely an
attribute capability number (att-cap-num) that identifies a
particular attribute capability by referring to attribute capability
numbers defined above and hence MUST be between 1 and 2^31-1 (both
included). The following example illustrates the above:
a=pcfg:1 a=-m:1,2,[3,4]|1,7,[5]
where
o "a=-m:1,2,[3,4]|1,7,[5]" is the attribute configuration list
o "-m" indicates to delete all attributes from the media
description of the actual configuration
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o "1,2,[3,4]" and "1,7,[5]" are both attribute capability lists.
The two lists are alternatives, since they are separated by a
vertical bar above
o "1", "2" and "7" are mandatory attribute capabilities
o "3", "4" and "5" are optional attribute capabilities
Note that in the example above, we have a single handle ("1") for
the potential configuration(s), but there are actually two different
potential configurations (separated by a vertical bar). This is done
for message size efficiency reasons, which is especially important
when we add other types of capabilities to the potential
configuration. If there is a need to provide a unique handle for
each, then separate "a=pcfg" attributes with different handles MUST
be used instead.
Each referenced attribute capability in the potential configuration
will result in the corresponding attribute name and its associated
value (contained inside the attribute capability) being added to the
resulting potential configuration SDP.
Alternative attribute capability lists are separated by a vertical
bar ("|"), the scope of which extends to the next alternative (i.e.,
"," has higher precedence than "|"). The alternatives are ordered by
preference with the most preferred listed first. In order for a
recipient of the SDP (e.g., an answerer receiving this in an offer)
to use this potential configuration, exactly one of the alternative
lists MUST be selected in its entirety. This requires that all
mandatory attribute capabilities referenced by the potential
configuration are supported with the attribute values provided.
Transport protocol configuration lists are included in a potential
configuration by use of the transport-protocol-config-list
parameter, which is defined by the following ABNF:
transport-protocol-config-list =
"t=" trpr-cap-num *(BAR trpr-cap-num)
trpr-cap-num = 1*DIGIT ; defined in [RFC5234]
Note that white space is not permitted within this production.
The trpr-cap-num refers to transport protocol capability numbers
defined above and hence MUST be between 1 and 2^31-1 (both
included). Alternative transport protocol capabilities are separated
by a vertical bar ("|"). The alternatives are ordered by preference
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with the most preferred listed first. If there are no transport
protocol capabilities included in a potential configuration at the
media level, the transport protocol information from the associated
"m=" line MUST be used. In order for a recipient of the SDP (e.g.,
an answerer receiving this in an offer) to use this potential
configuration, exactly one of the alternatives MUST be selected.
This requires that the transport protocol in question is supported.
In the presence of intermediaries (the existence of which may not
be known), care should be taken with assuming that the transport
protocol in the "m=" line will not be modified by an intermediary.
Use of an explicit transport protocol capability will guard
against capability negotiation implications of that.
Extension capabilities can be included in a potential configuration
as well by use of extension configuration lists. Such extension
configuration lists MUST adhere to the following ABNF:
extension-config-list= ["+"] ext-cap-name "="
ext-cap-list
ext-cap-name = 1*(ALPHA / DIGIT)
ext-cap-list = 1*VCHAR ; defined in [RFC5234]
Note that white space is not permitted within this production.
The ext-cap-name refers to the name of the extension capability and
the ext-cap-list is here merely defined as a sequence of visible
characters. The actual extension supported MUST refine both of these
further. For extension capabilities that merely need to be
referenced by a capability number, it is RECOMMENDED to follow a
structure similar to what has been specified above. Unsupported or
unknown potential extension configuration lists in a potential
configuration attribute MUST be ignored, unless they are prefixed
with the plus ("+") sign, which indicates that the extension is
mandatory and MUST be supported in order to use that potential
configuration.
The "creq" attribute and its associated rules can be used to
ensure that required extensions are supported in the first place.
Potential configuration attributes can be provided at the media
level only, however it is possible to reference capabilities
provided at either the session or media level. There are certain
semantic rules and restrictions associated with this:
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A (media level) potential configuration attribute in a given media
description MUST NOT reference a media-level capability provided in
a different media description; doing so invalidates that potential
configuration (note that a potential configuration attribute can
contain more than one potential configuration by use of
alternatives). A potential configuration attribute can however
reference a session-level capability. The semantics of doing so
depends on the type of capability. In the case of transport protocol
capabilities it has no particular implication. In the case of
attribute capabilities however, it does. More specifically, the
attribute name and value (provided within that attribute capability)
will be considered part of the resulting SDP for that particular
configuration at the *session* level. In other words, it will be as-
if that attribute was provided with that value at the session-level
in the first place. As a result, the base SDP Capability Negotiation
framework REQUIRES that potential configurations do not reference
any session-level attribute capabilities that contain media-level
attributes (since that would place a media-level attribute at the
session level). Extensions may modify this behavior, as long as it
is fully backwards compatible with the base specification.
Individual media streams perform capability negotiation
individually, and hence it is possible that one media stream (where
the attribute was part of a potential configuration) chose a
configuration without a session level attribute that was chosen by
another media stream. The session-level attribute however remains
"active" and applies to the entire resulting potential configuration
SDP. In theory, this is problematic if one or more session-level
attributes either conflicts with or potentially interacts with
another session-level or media-level attribute in an undefined
manner. In practice, such examples seem to be rare (at least with
the SDP attributes that had been defined at time of publication of
this document).
A related set of problems can occur if we need coordination
between session-level attributes from multiple media streams in
order for a particular functionality to work. The grouping
framework [RFC3388] is an example of this. If we use the SDP
Capability Negotiation framework to select a session-level group
attribute (provided as an attribute capability), and we require
two media descriptions to do this consistently, we could have a
problem. The FEC grouping semantics [RFC4756] is one example where
this in theory could cause problems, however in practice, it is
unclear that there is a significant problem with the grouping
semantics that had been defined at time of publication of this
document.
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Resolving the above issues in general requires inter-media stream
constraints and synchronized potential configuration processing;
this would add considerable complexity to the overall solution. In
practice, with the SDP attributes defined at time of publication of
this document, it does not seem to be a significant problem, and
hence the core SDP Capability Negotiation solution does not provide
a solution to this issue. Instead, it is RECOMMENDED that use of
session-level attributes in a potential configuration is avoided
when possible, and when not, that such use is examined closely for
any potential interaction issues. If interaction is possible, the
entity generating the SDP SHOULD NOT assume that well-defined
operation will occur at the receiving entity.
The session-level operation of extension capabilities is undefined:
Consequently, each new session-level extension capability defined
MUST specify the implication of making it part of a configuration at
the media level.
Below, we provide an example of the "a=pcfg" attribute in a complete
media description in order to properly indicate the supporting
attributes:
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
m=audio 53456 RTP/AVPF 0 18
a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=tcap:1 RTP/AVPF RTP/AVP RTP/SAVP RTP/SAVPF
a=pcfg:1 t=4|3 a=1
a=pcfg:8 t=1|2
We have two potential configuration attributes listed here. The
first one (and most preferred, since its configuration number is
"1") indicates that either of the profiles RTP/SAVPF or RTP/SAVP
(specified by the transport protocol capability numbers 4 and 3) can
be supported with attribute capability 1 (the "crypto" attribute);
RTP/SAVPF is preferred over RTP/SAVP since its capability number (4)
is listed first in the preferred potential configuration. Note that
although we have a single potential configuration attribute and
associated handle, we have two potential configurations.
The second potential configuration attribute indicates that the
RTP/AVPF or RTP/AVP profiles can be used, with RTP/AVPF being the
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preferred one. This non secure RTP alternative is the less preferred
one since its configuration number is "8". Again, note that we have
two potential configurations here and hence a total of four
potential configurations in the SDP above.
3.5.2. Actual Configuration Attribute
The actual configuration attribute identifies which of the potential
configurations from an offer SDP was selected and used as the actual
configuration to generate an answer SDP. This is done by including
the configuration number and the configuration lists (if any) from
the offer that were selected and used by the answerer in his
offer/answer procedure as follows:
o A selected attribute configuration MUST include the delete-
attributes and the known and supported parameters from the
selected alternative mo-att-cap-list (i.e., containing all
mandatory and all known and supported optional capability numbers
from the potential configuration). If delete-attributes were not
included in the potential configuration, they will of course not
be present here either.
o A selected transport protocol configuration MUST include the
selected transport protocol capability number.
o A selected potential extension configuration MUST include the
selected extension configuration parameters as specified for that
particular extension.
o When a configuration list contains alternatives (separated by
"|"), the selected configuration only MUST be provided.
Note that the selected configuration number and all selected
capability numbers used in the actual configuration attribute refer
to those from the offer; not the answer.
The answer may for example include capabilities as well to inform
the offerer of the answerers capabilities above and beyond the
negotiated configuration. The actual configuration attribute does
not refer to any of those answer capabilities though.
The Actual Configuration Attribute ("a=acfg") is defined as follows:
a=acfg: <config-number> [<sel-cfg-list>]
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where <config-number> is an integer between 1 and 2^31-1 (both
included). The attribute can be provided at the media-level only.
The "acfg" attribute adheres to the RFC 4566 "attribute" production,
with an att-value defined as follows:
att-value = config-number [1*WSP sel-cfg-list]
;config-number defined in Section 3.5.1.
sel-cfg-list = sel-cfg *(1*WSP sel-cfg)
sel-cfg = sel-attribute-config /
sel-transport-protocol-config /
sel-extension-config
sel-attribute-config =
"a=" [delete-attributes ":"] mo-att-cap-list
; defined in Section 3.5.1.
sel-transport-protocol-config =
"t=" trpr-cap-num ; defined in Section 3.5.1.
sel-extension-config =
ext-cap-name "=" 1*VCHAR ; defined in Section 3.5.1.
Note that white space is not permitted before the config-number.
The actual configuration ("a=acfg") attribute can be provided at the
media-level only. There MUST NOT be more than one occurrence of an
actual configuration attribute within a given media description.
Below, we provide an example of the "a=acfg" attribute (building on
the previous example with the potential configuration attribute):
v=0
o=- 24351 621814 IN IP4 192.0.2.2
s=
c=IN IP4 192.0.2.2
t=0 0
m=audio 54568 RTP/SAVPF 0
a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:WSJ+PSdFcGdUJShpX1ZjNzB4d1BINUAvLEw6UzF3|2^20|1:32
a=acfg:1 t=4 a=1
It indicates that the answerer used an offer consisting of potential
configuration number 1 with transport protocol capability 4 from the
offer (RTP/SAVPF) and attribute capability 1 (the "crypto"
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attribute). The answerer includes his own "crypto" attribute as
well.
3.6. Offer/Answer Model Extensions
In this section, we define extensions to the offer/answer model
defined in [RFC3264] to allow for potential configurations to be
included in an offer, where they constitute alternative offers that
may be accepted by the answerer instead of the actual
configuration(s) included in the "m=" line(s).
The procedures defined in the following subsections apply to both
unicast and multicast streams.
3.6.1. Generating the Initial Offer
An offerer that wants to use the SDP Capability Negotiation defined
in this document MUST include the following in the offer:
o Zero or more attribute capability attributes. There MUST be an
attribute capability attribute ("a=acap") as defined in Section3.4.1. for each attribute name and associated value (if any) that
needs to be indicated as a capability in the offer. Attribute
capabilities may be included irrespective of whether they are
referenced by a potential configuration or not.
Session-level attributes and associated values MUST be provided
in attribute capabilities at the session-level only, whereas
media-level attributes and associated values can be provided in
attribute capabilities at either the media-level or session-
level. Attributes that are allowed at either the session- or
media-level can be provided in attribute capabilities at either
level.
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o Zero or more transport protocol capability attributes. There MUST
be transport protocol capabilities as defined in Section 3.4.2.
with values for each transport protocol that needs to be
indicated as a capability in the offer. Transport protocol
capabilities may be included irrespective of whether they are
referenced by a potential configuration or not.
Transport protocol capabilities that apply to multiple media
descriptions SHOULD be provided at the session-level whereas
transport protocol capabilities that apply to a specific media
description ("m=" line) only, SHOULD be provided within that
particular media description. In either case, there MUST NOT be
more than a single "a=tcap" attribute at the session-level and a
single "a=tcap" attribute in each media description.
o Zero or more extension capability attributes. There MUST be one
or more extension capability attributes (as outlined in Section3.4.3. ) for each extension capability that is referenced by a
potential configuration. Extension capability attributes that are
not referenced by a potential configuration can be provided as
well.
o Zero or more potential configuration attributes. There MUST be
one or more potential configuration attributes ("a=pcfg"), as
defined in Section 3.5.1. , in each media description where
alternative potential configurations are to be negotiated. Each
potential configuration attribute MUST adhere to the rules
provided in Section 3.5.1. and the additional rules provided
below.
If the offerer requires support for more or extensions (besides the
base protocol defined here), then the offerer MUST include one or
more "a=creq" attributes as follows:
o If support for one or more capability negotiation extensions is
required for the entire session description, then option tags for
those extensions MUST be included in a single session-level
"creq" attribute.
o For each media description that requires support for one or more
capability negotiation extensions not listed at the session-
level, a single "creq" attribute containing all the required
extensions for that media description MUST be included within the
media description (in accordance with Section 3.3.2. ).
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Note that extensions that only need to be supported by a particular
potential configuration can use the "mandatory" extension prefix
("+") within the potential configuration (see Section 3.5.1. ).
The offerer SHOULD furthermore include the following:
o A supported capability negotiation extension attribute ("a=csup")
at the session-level and/or media-level as defined in Section3.3.2. for each capability negotiation extension supported by the
offerer and not included in a corresponding "a=creq" attribute
(i.e., at the session-level or in the same media description).
Option tags provided in a "a=csup" attribute at the session-level
indicate extensions supported for the entire session description,
whereas option tags provided in a "a=csup" attribute in a media
description indicate extensions supported for that particular
media description only.
Capabilities provided in an offer merely indicate what the offerer
is capable of doing. They do not constitute a commitment or even an
indication to use them. In contrast, each potential configuration
constitutes an alternative offer that the offerer would like to use.
The potential configurations MUST be used by the answerer to
negotiate and establish the session.
The offerer MUST include one or more potential configuration
attributes ("a=pcfg") in each media description where the offerer
wants to provide alternative offers (in the form of potential
configurations). Each potential configuration attribute in a given
media description MUST contain a unique configuration number and one
or more potential configuration lists, as described in Section3.5.1. Each potential configuration list MUST refer to capabilities
that are provided at the session-level or within that particular
media description; otherwise, the potential configuration is
considered invalid. The base SDP Capability Negotiation framework
REQUIRES that potential configurations do not reference any session-
level attribute capabilities that contain media-level only
attributes, however extensions may modify this behavior, as long as
it is fully backwards compatible with the base specification.
Furthermore, it is RECOMMENDED that potential configurations avoid
use of session-level capabilities whenever possible; refer to
Section 3.5.1.
The current actual configuration is included in the "m=" line (as
defined by [RFC3264]) and any associated parameters for the media
description (e.g., attribute ("a=") and bandwidth ("b=") lines).
Note that the actual configuration is by default the least-preferred
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configuration, and hence the answerer will seek to negotiate use of
one of the potential configurations instead. If the offerer wishes a
different preference for the actual configuration, the offerer MUST
include a corresponding potential configuration with the relevant
configuration number (which indicates the relative preference
between potential configurations); this corresponding potential
configuration should simply duplicate the actual configuration.
This can either be done implicitly (by not referencing any
capabilities), or explicitly (by providing and using capabilities
for the transport protocol and all the attributes that are part of
the actual configuration). The latter may help detect
intermediaries that modify the actual configuration but are not
SDP Capability Negotiation aware.
Per [RFC3264], once the offerer generates the offer, he must be
prepared to receive incoming media in accordance with that offer.
That rule applies here as well, but for the actual configurations
provided in the offer only: Media received by the offerer according
to one of the potential configurations MAY be discarded, until the
offerer receives an answer indicating what the actual selected
configuration is. Once that answer is received, incoming media MUST
be processed in accordance with the actual selected configuration
indicated and the answer received (provided the offer/answer
exchange completed successfully).
The above rule assumes that the offerer can determine whether
incoming media adheres to the actual configuration offered or one of
the potential configurations instead; this may not always be the
case. If the offerer wants to ensure he does not play out any
garbage, the offerer SHOULD discard all media received before the
answer SDP is received. Conversely, if the offerer wants to avoid
clipping, he should attempt to play any incoming media as soon as it
is received (at the risk of playing out garbage). For further
details, please refer to Section 3.9.
3.6.2. Generating the Answer
When receiving an offer, the answerer MUST check for the presence of
a required capability negotiation extension attribute ("a=creq")
provided at the session level. If one is found, then capability
negotiation MUST be performed. If none is found, then the answerer
MUST check each offered media description for the presence of a
required capability negotiation extension attribute ("a=creq") and
one or more potential configuration attributes ("a=pcfg").
Capability negotiation MUST be performed for each media description
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where either of those is present in accordance with the procedures
described below.
The answerer MUST first ensure that it supports any required
capability negotiation extensions:
o If a session-level "creq" attribute is provided, and it contains
an option-tag that the answerer does not support, then the
answerer MUST NOT use any of the potential configuration
attributes provided for any of the media descriptions. Instead,
the normal offer/answer procedures MUST continue as per
[RFC3264]. Furthermore, the answerer MUST include a session-level
supported capability negotiation extensions attribute ("a=csup")
with option tags for the capability negotiation extensions
supported by the answerer.
o If a media-level "creq" attribute is provided, and it contains an
option tag that the answerer does not support, then the answerer
MUST NOT use any of the potential configuration attributes
provided for that particular media description. Instead, the
offer/answer procedures for that media description MUST continue
as per [RFC3264] (SDP Capability Negotiation is still performed
for other media descriptions in the SDP). Furthermore, the
answerer MUST include a supported capability negotiation
extensions attribute ("a=csup") in that media description with
option tags for the capability negotiation extensions supported
by the answerer for that media description.
Assuming all required capability negotiation extensions are
supported, the answerer now proceeds as follows.
For each media description where capability negotiation is to be
performed (i.e. all required capability negotiation extensions are
supported and at least one valid potential configuration attribute
is present), the answerer MUST perform capability negotiation by
using the most preferred potential configuration that is valid to
the answerer, subject to any local policies. A potential
configuration is valid to the answerer if:
1. It is in accordance with the syntax and semantics provided in
Section 3.5.1.
2. It contains a configuration number that is unique within that
media description.
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3. All attribute capabilities referenced by the potential
configuration are valid themselves (as defined in Section 3.4.1.
) and each of them is provided either at the session-level or
within this particular media description. For session-level
attribute capabilities referenced, the attributes contained
inside them MUST NOT be media-level only attributes.
4. All transport protocol capabilities referenced by the potential
configuration are valid themselves (as defined in Section 3.4.2.
) and each of them is furthermore provided either at the session-
level or within this particular media description.
5. All extension capabilities referenced by the potential
configuration and supported by the answerer are valid themselves
(as defined by that particular extension) and each of them are
furthermore provided either at the session-level or within this
particular media description. Unknown or unsupported extension
capabilities MUST be ignored, unless they are prefixed with the
plus ("+") sign, which indicates that the extension MUST be
supported in order to use that potential configuration. If the
extension is not supported, that potential configuration is not
valid to the answerer.
The most preferred valid potential configuration in a media
description is the valid potential configuration with the lowest
configuration number. The answerer MUST now process the offer for
that media stream based on the most preferred valid potential
configuration. Conceptually, this entails the answerer constructing
an (internal) offer that consists of the actual configuration offer
SDP, with the following changes for each media stream offered:
o If a transport protocol capability is included in the potential
configuration, then it replaces the transport protocol provided
in the "m=" line for that media description.
o If attribute capabilities are present with a delete-attributes
session indication ("-s"), then all session-level attributes from
the actual configuration SDP MUST be deleted in accordance with
the procedures in Section 3.5.1. If attribute capabilities are
present with a delete-attributes media indication ("-m"), then
all attributes from the actual configuration SDP inside this
media description MUST be deleted.
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o If a session-level attribute capability is included, the
attribute (and its associated value, if any) contained in it MUST
be added to the resulting SDP. All such added session-level
attributes MUST be listed before the session-level attributes
that were initially present in the SDP. Furthermore, the added
session-level attributes MUST be added in the order they were
provided in the potential configuration (see also Section 3.5.1.
).
This allows for attributes with implicit preference ordering
to be added in the desired order; the "crypto" attribute
[RFC4568] is one such example.
o If a media-level attribute capability is included, then the
attribute (and its associated value, if any) MUST be added to the
resulting SDP within the media description in question. All such
added media-level attributes MUST be listed before the media-
level attributes that were initially present in the SDP in the
media description in question. Furthermore, the added media-level
attributes MUST be added in the order they were provided in the
potential configuration (see also Section 3.5.1. ).
o If a supported extension capability is included, then it MUST be
processed in accordance with the rules provided for that
particular extension capability.
Note that a transport protocol from the potential configuration
replaces the transport protocol in the actual configuration, but an
attribute capability from the potential configuration is simply
added to the actual configuration. In some cases, this can result in
having one or more meaningless attributes in the resulting potential
configuration SDP, or worse, ambiguous or potentially even illegal
attributes. Use of delete-attributes for the session and/or media
level attributes MUST be done to avoid such scenarios. Nevertheless,
it is RECOMMENDED that implementations ignore meaningless attributes
that may result from potential configurations.
For example, if the actual configuration was using Secure RTP and
included an "a=crypto" attribute for the SRTP keying material,
then use of a potential configuration that uses plain RTP would
make the "crypto" attribute meaningless. The answerer may or may
not ignore such a meaningless attribute. The offerer can here
ensure correct operation by using delete-attributes to remove the
crypto attribute (but will then need to provide attribute
capabilities to reconstruct the SDP with the necessary attributes
deleted, e.g. rtpmaps).
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Please refer to Section 3.6.2.1. for examples of how the answerer
may conceptually "see" the resulting offered alternative potential
configurations.
The answerer MUST check that he supports all mandatory attribute
capabilities from the potential configuration (if any), the
transport protocol capability (if any) from the potential
configuration, and all mandatory extension capabilities from the
potential configuration (if any). If he does not, the answerer MUST
proceed to the second-most preferred valid potential configuration
for the media description, etc.
o In the case of attribute capabilities, support implies that the
attribute name contained in the capability is supported and it
can (and will) be negotiated successfully in the offer/answer
exchange with the value provided. This does not necessarily imply
that the value provided is supported in its entirety. For
example, the "a=fmtp" parameter is often provided with one or
more values in a list, where the offerer and answerer negotiate
use of some subset of the values provided. Other attributes may
include mandatory and optional parts to their values; support for
the mandatory part is all that is required here.
A side-effect of the above rule is that whenever an "fmtp" or
"rtpmap" parameter is provided as a mandatory attribute
capability, the corresponding media format (codec) must be
supported and use of it negotiated successfully. If this is
not the offerer's intent, the corresponding attribute
capabilities must be listed as optional instead.
o In the case of transport protocol capabilities, support implies
that the transport protocol contained in the capability is
supported and the transport protocol can (and will) be negotiated
successfully in the offer/answer exchange.
o In the case of extension capabilities, the extension MUST define
the rules for when the extension capability is considered
supported and those rules MUST be satisfied.
If the answerer has exhausted all potential configurations for the
media description, without finding a valid one that is also
supported, then the answerer MUST process the offered media stream
based on the actual configuration plus any session-level attributes
added by a valid and supported potential configuration from another
media description in the offered SDP.
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The above process describes potential configuration selection as a
per media stream process. Inter-media stream coordination of
selected potential configurations however is required in some cases.
First of all, session-level attributes added by a potential
configuration for one media description MUST NOT cause any problems
for potential configurations selected by other media descriptions in
the offer SDP. If the session-level attributes are mandatory, then
those session-level attributes MUST furthermore be supported by the
session as a whole (i.e., all the media descriptions if relevant).
As mentioned earlier, this adds additional complexity to the overall
processing and hence it is RECOMMENDED not to use session-level
attribute capabilities in potential configurations, unless
absolutely necessary.
Once the answerer has selected a valid and supported offered
potential configuration for all of the media streams (or has fallen
back to the actual configuration plus any added session attributes),
the answerer MUST generate a valid answer SDP based on the selected
potential configuration SDP, as "seen" by the answerer (see Section3.6.2.1. for examples). Furthermore, if the answerer selected one of
the potential configurations in a media description, the answerer
MUST include an actual configuration attribute ("a=acfg") within
that media description. The "a=acfg" attribute MUST identify the
configuration number for the selected potential configuration as
well as the actual parameters that were used from that potential
configuration; if the potential configuration included alternatives,
the selected alternatives only MUST be included. Only the known and
supported parameters will be included. Unknown or unsupported
parameters MUST NOT be included in the actual configuration
attribute. In the case of attribute capabilities, only the known and
supported capabilities are included; unknown or unsupported
attribute capabilities MUST NOT be included.
If the answerer supports one or more capability negotiation
extensions that were not included in a required capability
negotiation extensions attribute in the offer, then the answerer
SHOULD furthermore include a supported capability negotiation
attribute ("a=csup") at the session-level with option tags for the
extensions supported across media streams. Also, if the answerer
supports one or more capability negotiation extensions for
particular media descriptions only, then a supported capability
negotiation attribute with those option-tags SHOULD be included
within each relevant media description. The required capability
negotiation attribute ("a=creq") MUST NOT be used in an answer.
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The offerer's originally provided actual configuration is contained
in the offer media description's "m=" line (and associated
parameters). The answerer MAY send media to the offerer in
accordance with that actual configuration as soon as it receives the
offer, however it MUST NOT send media based on that actual
configuration if it selects an alternative potential configuration.
If the answerer selects one of the potential configurations, then
the answerer MAY immediately start to send media to the offerer in
accordance with the selected potential configuration, however the
offerer MAY discard such media or play out garbage until the offerer
receives the answer. Please refer to section 3.9. for additional
considerations and possible alternative solutions outside the base
SDP Capability Negotiation framework.
If the offerer selected a potential configuration instead of the
actual configuration, then it is RECOMMENDED that the answerer sends
back an answer SDP as soon as possible. This minimizes the risk of
having media discarded or played out as garbage by the offerer. In
the case of SIP [RFC3261] without any extensions, this implies that
if the offer was received in an INVITE message, then the answer SDP
should be provided in the first non-100 provisional response sent
back (per RFC3261, the answer would need to be repeated in the 200
response as well, unless a relevant extension such as [RFC3262] is
being used).
3.6.2.1. Example Views of Potential Configurations
The following examples illustrate how the answerer may conceptually
"see" a potential configuration. Consider the following offered SDP:
v=0
o=alice 2891092738 2891092738 IN IP4 lost.example.com
s=
t=0 0
c=IN IP4 lost.example.com
a=tool:foo
a=acap:1 key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
a=tcap:1 RTP/SAVP RTP/AVP
m=audio 59000 RTP/AVP 98
a=rtpmap:98 AMR/8000
a=acap:2 crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=pcfg:1 t=1 a=1|2
m=video 52000 RTP/AVP 31
a=rtpmap:31 H261/90000
a=acap:3 crypto:1 AES_CM_128_HMAC_SHA1_80
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inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
a=pcfg:1 t=1 a=1|3
This particular SDP offers an audio stream and a video stream, each
of which can either use plain RTP (actual configuration) or secure
RTP (potential configuration). Furthermore, two different keying
mechanisms are offered, namely session-level Key Management
Extensions using MIKEY (attribute capability 1) and media-level SDP
Security Descriptions (attribute capabilities 2 and 3). There are
several potential configurations here, however, below we show the
one the answerer "sees" when using potential configuration 1 for
both audio and video, and furthermore using attribute capability 1
(MIKEY) for both (we have removed all the capability negotiation
attributes for clarity):
v=0
o=alice 2891092738 2891092738 IN IP4 lost.example.com
s=
t=0 0
c=IN IP4 lost.example.com
a=tool:foo
a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
m=audio 59000 RTP/SAVP 98
a=rtpmap:98 AMR/8000
m=video 52000 RTP/SAVP 31
a=rtpmap:31 H261/90000
Note that the transport protocol in the media descriptions indicate
use of secure RTP.
Below, we show the offer the answerer "sees" when using potential
configuration 1 for both audio and video and furthermore using
attribute capability 2 and 3 respectively (SDP security
descriptions) for the audio and video stream - note the order in
which the resulting attributes are provided:
v=0
o=alice 2891092738 2891092738 IN IP4 lost.example.com
s=
t=0 0
c=IN IP4 lost.example.com
a=tool:foo
m=audio 59000 RTP/SAVP 98
a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
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a=rtpmap:98 AMR/8000
m=video 52000 RTP/SAVP 31
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
a=rtpmap:31 H261/90000
Again, note that the transport protocol in the media descriptions
indicate use of secure RTP.
And finally, we show the offer the answerer "sees" when using
potential configuration 1 with attribute capability 1 (MIKEY) for
the audio stream, and potential configuration 1 with attribute
capability 3 (SDP security descriptions) for the video stream:
v=0
o=alice 2891092738 2891092738 IN IP4 lost.example.com
s=
t=0 0
c=IN IP4 lost.example.com
a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
a=tool:foo
m=audio 59000 RTP/SAVP 98
a=rtpmap:98 AMR/8000
m=video 52000 RTP/SAVP 31
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
a=rtpmap:31 H261/90000
3.6.3. Offerer Processing of the Answer
When the offerer attempted to use SDP Capability Negotiation in the
offer, the offerer MUST examine the answer for actual use of SDP
Capability Negotiation.
For each media description where the offerer included a potential
configuration attribute ("a=pcfg"), the offerer MUST first examine
that media description for the presence of an actual configuration
attribute ("a=acfg"). If an actual configuration attribute is not
present in a media description, then the offerer MUST process the
answer SDP for that media stream per the normal offer/answer rules
defined in [RFC3264]. However, if one is found, the offerer MUST
instead process the answer as follows:
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o The actual configuration attribute specifies which of the
potential configurations was used by the answerer to generate the
answer for this media stream. This includes all the supported
attribute capabilities and the transport capabilities referenced
by the potential configuration selected, where the attribute
capabilities have any associated delete-attributes included.
Extension capabilities supported by the answerer are included as
well.
o The offerer MUST now process the answer in accordance with the
rules in [RFC3264], except that it must be done as if the offer
consisted of the selected potential configuration instead of the
original actual configuration, including any transport protocol
changes in the media ("m=") line(s), attributes added and deleted
by the potential configuration at the media and session level,
and any extensions used.
If the offer/answer exchange was successful, and if the answerer
selected one of the potential configurations from the offer as the
actual configuration, and the selected potential configuration
differs from the actual configuration in the offer (the "m=", "a=",
etc. lines), then the offerer SHOULD initiate another offer/answer
exchange. This second offer/answer exchange will not modify the
session in any way, however it will help intermediaries (e.g.
middleboxes), that look at the SDP but do not support the capability
negotiation extensions, understand the details of the media
stream(s) that were actually negotiated. This new offer MUST contain
the selected potential configuration as the actual configuration,
i.e., with the actual configuration used in the "m=" line and any
other relevant attributes, bandwidth parameters, etc.
Note that, per normal offer/answer rules, the second offer/answer
exchange still needs to update the version number in the "o=" line
((<sess-version> in [RFC4566]). Attribute lines carrying keying
material SHOULD repeat the keys from the previous offer, unless re-
keying is necessary, e.g. due to a previously forked SIP INVITE
request. Please refer to Section 3.12. for additional considerations
related to intermediaries.
3.6.4. Modifying the Session
Capabilities and potential configurations may be included in
subsequent offers as defined in [RFC3264], Section 8. The procedure
for doing so is similar to that described above with the answer
including an indication of the actual selected configuration used by
the answerer.
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If the answer indicates use of a potential configuration from the
offer, then the guidelines provided in Section 3.6.3. for doing a
second offer/answer exchange using that potential configuration as
the actual configuration apply.
3.7. Interactions with ICE
Interactive Connectivity Establishment (ICE) [ICE] provides a
mechanism for verifying connectivity between two endpoints by
sending STUN messages directly between the media endpoints. The
basic ICE specification [ICE] is defined to support UDP-based
connectivity only, however it allows for extensions to support other
transport protocols, such as TCP, which is being specified in
[ICETCP]. ICE defines a new "a=candidate" attribute, which, among
other things, indicates the possible transport protocol(s) to use
and then associates a priority with each of them. The most preferred
transport protocol that *successfully* verifies connectivity will
end up being used.
When using ICE, it is thus possible that the transport protocol that
will be used differs from what is specified in the "m=" line. Since
both ICE and SDP Capability Negotiation may specify alternative
transport protocols, there is a potentially unintended interaction
when using these together.
We provide the following guidelines for addressing that.
There are two basic scenarios to consider:
1) A particular media stream can run over different transport
protocols (e.g. UDP, TCP, or TCP/TLS), and the intent is simply to
use the one that works (in the preference order specified).
2) A particular media stream can run over different transport
protocols (e.g. UDP, TCP, or TCP/TLS) and the intent is to have the
negotiation process decide which one to use (e.g. T.38 over TCP or
UDP).
In scenario 1, there should be ICE "a=candidate" attributes for UDP,
TCP, etc. but otherwise nothing special in the potential
configuration attributes to indicate the desire to use different
transport protocols (e.g. UDP, or TCP). The ICE procedures
essentially cover the capability negotiation required (by having the
answerer select something it supports and then use of trial and
error connectivity checks).
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Scenario 2 does not require a need to support or use ICE. Instead,
we simply use transport protocol capabilities and potential
configuration attributes to indicate the desired outcome.
The scenarios may be combined, e.g. by offering potential
configuration alternatives where some of them can support one
transport protocol only (e.g. UDP), whereas others can support
multiple transport protocols (e.g. UDP or TCP). In that case, there
is a need for tight control over the ICE candidates that will be
used for a particular configuration, yet the actual configuration
may want to use all of the ICE candidates. In that case, the ICE
candidate attributes can be defined as attribute capabilities and
the relevant ones should then be included in the proper potential
configurations (for example candidate attributes for UDP only for
potential configurations that are restricted to UDP, whereas there
could be candidate attributes for UDP, TCP, and TCP/TLS for
potential configurations that can use all three). Furthermore, use
of the delete-attributes in a potential configuration can be used to
ensure that ICE will not end up using a transport protocol that is
not desired for a particular configuration.
SDP Capability Negotiation recommends use of a second offer/answer
exchange when the negotiated actual configuration was one of the
potential configurations from the offer (see Section 3.6.3. ).
Similarly, ICE requires use of a second offer/answer exchange if the
chosen candidate is not the same as the one in the m/c-line from the
offer. When ICE and capability negotiation are used at the same
time, the two secondary offer/answer exchanges SHOULD be combined to
a single one.
3.8. Interactions with SIP Option Tags
SIP [RFC3261] allows for SIP extensions to define a SIP option tag
that identifies the SIP extension. Support for one or more such
extensions can be indicated by use of the SIP Supported header, and
required support for one or more such extensions can be indicated by
use of the SIP Require header. The "a=csup" and "a=creq" attributes
defined by the SDP Capability Negotiation framework are similar,
except that support for these two attributes by themselves cannot be
guaranteed (since they are specified as extensions to the SDP
specification [RFC4566] itself).
SIP extensions with associated option tags can introduce
enhancements to not only SIP, but also SDP. This is for example the
case for SIP preconditions defined in [RFC3312]. When using SDP
Capability Negotiation, some potential configurations may include
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certain SDP extensions, whereas others may not. Since the purpose of
the SDP Capability Negotiation is to negotiate a session based on
the features supported by both sides, use of the SIP Require header
for such extensions may not produce the desired result. For example,
if one potential configuration requires SIP preconditions support,
another does not, and the answerer does not support preconditions,
then use of the SIP Require header for preconditions would result in
a session failure, in spite of the fact that a valid and supported
potential configuration was included in the offer.
In general, this can be alleviated by use of mandatory and optional
attribute capabilities in a potential configuration. There are
however cases where permissible SDP values are tied to the use of
the SIP Require header. SIP preconditions [RFC3312] is one such
example, where preconditions with a "mandatory" strength-tag can
only be used when a SIP Require header with the SIP option tag
"precondition" is included. Future SIP extensions that may want to
use the SDP Capability Negotiation framework should avoid such
coupling.
3.9. Processing Media before Answer
The offer/answer model requires an offerer to be able to receive
media in accordance with the offer prior to receiving the answer.
This property is retained with the SDP Capability Negotiation
extensions defined here, but only when the actual configuration is
selected by the answerer. If a potential configuration is chosen, it
is permissible for the offerer to not process any media received
before the answer is received. This may lead to clipping.
Consequently, the SDP Capability Negotiation framework recommends
sending back an answer SDP as soon as possible.
The issue can be resolved by introducing a three-way handshake. In
the case of SIP, this can for example be done by defining a
precondition [RFC3312] for capability negotiation (or use an
existing precondition that is known to generate a second
offer/answer exchange before proceeding with the session). However,
preconditions are often viewed as complicated to implement and they
may add to overall session establishment delay by requiring an extra
offer/answer exchange.
An alternative three-way handshake can be performed by use of ICE
[ICE]. When ICE is being used, and the answerer receives a STUN
Binding Request for any one of the accepted media streams from the
offerer, the answerer knows the offer has received his answer. At
that point, the answerer knows that the offerer will be able to
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process incoming media according to the negotiated configuration and
hence he can start sending media without the risk of the offerer
either discarding it or playing garbage.
In some use cases a three-way handshake is not needed. An example is
when the offerer does not need information from the answer, such as
keying material in the SDP, in order to process incoming media. The
SDP Capability Negotiation framework does not define any such
solutions, however extensions may do so. For example, one technique
proposed for best-effort SRTP in [BESRTP] is to provide different
RTP payload type mappings for different transport protocols used,
outside of the actual configuration, while still allowing them to be
used by the answerer (exchange of keying material is still needed,
e.g. inband). The basic SDP Capability Negotiation framework defined
here does not include the ability to do so, however extensions that
enable that may be defined.
3.10. Indicating Bandwidth Usage
The amount of bandwidth used for a particular media stream depends
on the negotiated codecs, transport protocol and other parameters.
For example use of Secure RTP [RFC3711] with integrity protection
requires more bandwidth than plain RTP [RFC3551]. SDP defines the
bandwidth ("b=") parameter to indicate the proposed bandwidth for
the session or media stream.
In SDP as defined by [RFC4566], each media description contains one
transport protocol and one or more codecs. When specifying the
proposed bandwidth, the worst case scenario must be taken into
account, i.e., use of the highest bandwidth codec provided, the
transport protocol indicated, and the worst case (bandwidth-wise)
parameters that can be negotiated (e.g., a 32-bit HMAC or an 80-bit
HMAC).
The core SDP capability negotiation framework does not provide a way
to negotiate bandwidth parameters. The issue thus remains, however
it is potentially worse than with SDP per [RFC4566], since it is
easier to negotiate additional codecs, and furthermore possible to
negotiate different transport protocols. The recommended approach
for addressing this is the same as for plain SDP; the worst case
(now including potential configurations) needs to be taken into
account when specifying the bandwidth parameters in the actual
configuration. This can make the bandwidth value less accurate than
in SDP per [RFC4566] (due to potential greater variability in the
potential configuration bandwidth use). Extensions can be defined to
address this shortcoming. Also, the Transport Independent
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Application Specific Maximum (TIAS) bandwidth type defined in
[RFC3890] can be used to alleviate bandwidth variability concerns
due to different transport protocols.
Note, that when using RTP retransmission [RFC4588] with the RTCP-
based feedback profile [RFC4585] (RTP/AVPF), the retransmitted
packets are part of the media stream bandwidth when using SSRC-
multiplexing. If a feedback based protocol is offered as the actual
configuration transport protocol, a non-feedback based protocol is
offered as a potential configuration transport protocol and ends up
being used, the actual bandwidth usage may be lower than the
indicated bandwidth value in the offer (and vice versa).
3.11. Dealing with Large Number of Potential Configurations
When using the SDP Capability Negotiation, it is easy to generate
offers that contain a large number of potential configurations. For
example, in the offer:
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
m=audio 53456 RTP/AVP 0 18
a=tcap:1 RTP/SAVPF RTP/SAVP RTP/AVPF
a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_80
inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
FEC_ORDER=FEC_SRTP
a=acap:2 key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
a=acap:3 rtcp-fb:0 nack
a=pcfg:1 t=1 a=1,3|2,3
a=pcfg:2 t=2 a=1|2
a=pcfg:3 t=3 a=3
we have 5 potential configurations on top of the actual
configuration for a single media stream. Adding an extension
capability with just two alternatives for each would double that
number (to 10), and doing the equivalent with two media streams
would again double that number (to 20). While it is easy (and
inexpensive) for the offerer to generate such offers, processing
them at the answering side may not be. Consequently, it is
RECOMMENDED that offerers do not create offers with unnecessarily
large number of potential configurations in them.
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On the answering side, implementers MUST take care to avoid
excessive memory and CPU consumption. For example, a naive
implementation that first generates all the valid potential
configuration SDPs internally, could find itself being memory
exhausted, especially if it supports a large number of endpoints.
Similarly, a naive implementation that simply performs iterative
trial-and-error processing on each possible potential configuration
SDP (in the preference order specified) could find itself being CPU
constrained. An alternative strategy is to prune the search space
first by discarding the set of offered potential configurations
where the transport protocol indicated (if any) is not supported,
and/or one or more mandatory attribute capabilities (if any) are
either not supported or not valid. Potential configurations with
unsupported mandatory extension configurations in them can be
discarded as well.
3.12. SDP Capability Negotiation and Intermediaries
An intermediary is here defined as an entity between a SIP user
agent A and a SIP user agent B, that need to perform some kind of
processing on the SDP exchanged between A and B, in order for the
session establishment to operate as intended. Examples of such
intermediaries include Session Border Controllers (SBCs) that may
perform media relaying, Proxy Call Session Control Functions (P-
CSCF) that may authorize use of a certain amount of network
resources (bandwidth), etc. The presence and design of such
intermediaries may not follow the "Internet" model or the SIP
requirements for proxies (which are not supposed to look in message
bodies such as SDP), however they are a fact of life in some
deployment scenarios and hence deserve consideration.
If the intermediary needs to understand the characteristics of the
media sessions being negotiated, e.g. the amount of bandwidth used
or the transport protocol negotiated, then use of the SDP Capability
Negotiation framework may impact them. For example, some
intermediaries are known to disallow answers where the transport
protocol differs from the one in the offer. Use of the SDP
Capability Negotiation framework in the presence of such
intermediaries could lead to session failures. Intermediaries that
need to authorize use of network resources based on the negotiated
media stream parameters are affected as well. If they inspect only
the offer, then they may authorize parameters assuming a different
transport protocol, codecs, etc. than what is actually being
negotiated. For these, and other, reasons it is RECOMMENDED that
implementers of intermediaries add support for the SDP Capability
Negotiation framework.
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The SDP Capability Negotiation framework itself attempts to help out
these intermediaries as well, by optionally performing a second
offer/answer exchange when use of a potential configuration has been
negotiated (see Section 3.6.3. ). However, there are several
limitations with this approach. First of all, the second
offer/answer exchange is not required and hence may not be
performed. Secondly, the intermediary may refuse the initial answer,
e.g. due to perceived transport protocol mismatch. Thirdly, the
strategy is not foolproof, since the offer/answer procedures
[RFC3264] leave the original offer/answer exchange in effect when a
subsequent one fails; consider the following example:
1. Offerer generates an SDP offer with the actual configuration
specifying a low bandwidth configuration (e.g. plain RTP) and a
potential configuration specifying a high(er) bandwidth
configuration (e.g. secure RTP with integrity).
2. An intermediary (e.g. an SBC or P-CSCF), that does not support
SDP Capability Negotiation, authorizes the session based on the
actual configuration it sees in the SDP.
3. The answerer chooses the high(er) bandwidth potential
configuration and generates an answer SDP based on that.
4. The intermediary passes through the answer SDP.
5. The offerer sees the accepted answer, and generates an updated
offer that contains the selected potential configuration as the
actual configuration. In other words, the high(er) bandwidth
configuration (which has already been negotiated successfully) is
now the actual configuration in the offer SDP.
6. The intermediary sees the new offer, however it does not
authorize the use of the high(er) bandwidth configuration, and
consequently generates a rejection message to the offerer.
7. The offerer receives the rejected offer.
After step 7, per RFC 3264, the offer/answer exchange that completed
in step 5 remains in effect, however the intermediary may not have
authorized the necessary network resources and hence the media
stream may experience quality issues. The solution to this problem
is to upgrade the intermediary to support the SDP Capability
Negotiation framework.
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The core SDP Capability Negotiation framework defines transport
capabilities and attribute capabilities. Media capabilities, which
can be used to describe media formats and their associated
parameters, are not defined in this document, however the "rtpmap"
and "fmtp" attributes can nevertheless be used as attribute
capabilities. Using such attribute capabilities in a potential
configuration requires a bit of care though.
The rtpmap parameter binds an RTP payload type to a media format
(e.g. codec). While it is possible to provide rtpmaps for payload
types not found in the corresponding "m=" line, such rtpmaps provide
no value in normal offer/answer exchanges, since only the payload
types found in the "m=" line are part of the offer (or answer). This
applies to the core SDP Capability Negotiation framework as well:
Only the media formats (e.g. RTP payload types) provided in the "m="
line are actually offered; inclusion of rtpmap attributes with other
RTP payload types in a potential configuration does not change this
fact and hence they do not provide any useful information there.
They may still be useful as pure capabilities though (outside a
potential configuration) in order to inform a peer of additional
codecs supported.
It is possible to provide an rtpmap attribute capability with a
payload type mapping to a different codec than a corresponding
actual configuration "rtpmap" attribute for the media description
has. Such practice is permissible as a way of indicating a
capability. If that capability is included in a potential
configuration, then delete-attributes (see Section 3.5.1. ) MUST be
used to ensure that there is not multiple rtpmap attributes for the
same payload type in a given media description (which would not be
allowed by SDP [RFC4566]).
Similar considerations and rules apply to the "fmtp" attribute. An
fmtp attribute capability for a media format not included in the
"m=" line is useless in a potential configuration (but may be useful
as a capability by itself). An fmtp attribute capability in a
potential configuration for a media format that already has an fmtp
attribute in the actual configuration may lead to multiple fmtp
format parameters for that media format and that is not allowed by
SDP [RFC4566]. The delete-attributes MUST be used to ensure that
there is not multiple fmtp attributes for a given media format in a
media description.
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Extensions to the core SDP Capability Negotiation framework may
change the above behavior.
3.13.2. Direction Attributes
SDP defines the "inactive", "sendonly", "recvonly", and "sendrecv"
direction attributes. The direction attributes can be applied at
either the session-level or the media-level. In either case, it is
possible to define attribute capabilities for these direction
capabilities; if used by a potential configuration, the normal
offer/answer procedures still apply. For example, if an offered
potential configuration includes the "sendonly" direction attribute,
and it is selected as the actual configuration, then the answer MUST
include a corresponding "recvonly" (or "inactive") attribute.
3.14. Relationship to RFC 3407RFC 3407 defines capability descriptions with limited abilities to
describe attributes, bandwidth parameters, transport protocols and
media formats. RFC 3407 does not define any negotiation procedures
for actually using those capability descriptions.
This document defines new attributes for describing attribute
capabilities and transport capabilities. It also defines procedures
for using those capabilities as part of an offer/answer exchange. In
contrast to RFC 3407, this document does not define bandwidth
parameters, and it also does not define how to express ranges of
values. Extensions to this document may be defined in order to fully
cover all the capabilities provided by RFC 3407 (for example more
general media capabilities).
It is RECOMMENDED that implementations use the attributes and
procedures defined in this document instead of those defined in
[RFC3407]. If capability description interoperability with legacy
RFC 3407 implementations is desired, implementations MAY include
both RFC 3407 capability descriptions and capabilities defined by
this document. The offer/answer negotiation procedures defined in
this document will not use the RFC 3407 capability descriptions.
4. Examples
In this section, we provide examples showing how to use the SDP
Capability Negotiation.
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a=pcfg:3 t=3 a=[2]
The "m=" line indicates that Alice is offering to use plain RTP with
PCMU or G.729. The capabilities are provided by the "a=tcap" and
"a=acap" attributes. The "tcap" capability indicates that Secure
RTP with RTCP-Based feedback (RTP/SAVPF), Secure RTP (RTP/SAVP), and
RTP with RTCP-Based feedback are supported. The first "acap"
attribute provides an attribute capability with a handle of 1. The
capability is a "crypto" attribute, which provides the keying
material for SRTP using SDP security descriptions [RFC4568]. The
second "acap" attribute provides an attribute capability with a
handle of 2. The capability is an "rtcp-fb" attribute, which is used
by the RTCP-based feedback profiles to indicate that payload type 0
(PCMU) supports feedback type "nack". The "a=pcfg" attributes
provide the potential configurations included in the offer by
reference to the capabilities. There are three potential
configurations:
o Potential configuration 1, which is the most preferred potential
configuration specifies use of transport protocol capability 1
(RTP/SAVPF) and attribute capabilities 1 (the "crypto" attribute)
and 2 (the "rtcp-fb" attribute). Support for the first one is
mandatory whereas support for the second one is optional.
o Potential configuration 2, which is the second most preferred
potential configuration specifies use of transport protocol
capability 2 (RTP/SAVP) and mandatory attribute capability 1 (the
"crypto" attribute).
o Potential configuration 3, which is the least preferred potential
configuration (but the second least preferred configuration
overall, since the actual configuration provided by the "m=" line
is always the least preferred configuration), specifies use of
transport protocol capability 3 (RTP/AVPF) and optional attribute
capability 2 (the "rtcp-fb" attribute).
Bob receives the SDP offer from Alice. Bob does not support any
secure RTP profiles, however he supports plain RTP and RTP with
RTCP-based feedback, as well as the SDP Capability Negotiation
extensions, and hence he accepts the potential configuration for RTP
with RTCP-based feedback provided by Alice:
v=0
o=- 24351 621814 IN IP4 192.0.2.2
s=
c=IN IP4 192.0.2.2
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t=0 0
m=audio 54568 RTP/AVPF 0 18
a=rtcp-fb:0 nack
a=acfg:1 t=3 a=[2]
Bob includes the "a=acfg" attribute in the answer to inform Alice
that he based his answer on an offer containing the potential
configuration with transport protocol capability 3 and optional
attribute capability 2 from the offer SDP (i.e. the RTP/AVPF profile
using the "rtcp-fb" value provided). Bob also includes an "rtcp-fb"
attribute with the value "nack" value for RTP payload type 0.
When Alice receives Bob's answer, session negotiation has completed,
however Alice nevertheless chooses to generate a new offer using the
actual configuration. This is done purely to assist any
intermediaries that may reside between Alice and Bob but do not
support the SDP Capability Negotiation framework (and hence may not
understand the negotiation that just took place):
Alice's updated offer includes only RTP/AVPF, and it is not using
the SDP Capability Negotiation framework (Alice could have included
the capabilities as well if she wanted to):
v=0
o=- 25678 753850 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
m=audio 53456 RTP/AVPF 0 18
a=rtcp-fb:0 nack
The "m=" line now indicates that Alice is offering to use RTP with
RTCP-based feedback and using PCMU or G.729. The "rtcp-fb"
attribute provides the feedback type "nack" for payload type 0 again
(but as part of the actual configuration).
Bob receives the SDP offer from Alice, which he accepts, and then
generates an answer to Alice:
v=0
o=- 24351 621815 IN IP4 192.0.2.2
s=
c=IN IP4 192.0.2.2
t=0 0
m=audio 54568 RTP/AVPF 0 18
a=rtcp-fb:0 nack
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Bob includes the same "rtcp-fb" attribute as before, and the session
proceeds without change. Although Bob did not include any
capabilities in his answer, he could have done so if he wanted to.
Note that in this particular example, the answerer supported the SDP
Capability Negotiation framework and hence the attributes and
procedures defined here, however had he not, the answerer would
simply have ignored the new attributes received in step 1 and
accepted the offer to use normal RTP. In that case, the following
answer would have been generated in step 2 instead:
v=0
o=- 24351 621814 IN IP4 192.0.2.2
s=
c=IN IP4 192.0.2.2
t=0 0
m=audio 54568 RTP/AVP 0 18
4.2. Best-Effort SRTP with Session-Level MIKEY and Media Level Security Descriptions
The following example illustrates how to use the SDP Capability
Negotiation extensions to support so-called Best-Effort Secure RTP
as well as alternative keying mechanisms, more specifically MIKEY
[RFC3830] and SDP Security Descriptions. The offerer (Alice) wants
to establish an audio and video session. Alice prefers to use
session-level MIKEY as the key management protocol, but supports SDP
security descriptions as well.
The example is illustrated by the offer/answer exchange below, where
Alice sends an offer to Bob:
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attribute capabilities is mandatory. There are three potential
configurations for the video stream.
o The first configuration with configuration number 1 uses
transport capability 1 (RTP/SAVPF) with either attribute
capabilities 1 and 4 (session-level MIKEY and the "rtcp-fb"
attribute) or attribute capabilities 3 and 4 (SDP security
descriptions and the "rtcp-fb" attribute). In this example, the
offerer insists on not only the keying mechanism being supported,
but also that the "rtcp-fb" attribute is supported with the value
indicated. Consequently, all the attribute capabilities are
marked as mandatory in this potential configuration.
o The second configuration with configuration number 2 uses
transport capability 2 (RTP/SAVP) and either attribute capability
1 (session-level MIKEY) or attribute capability 3 (SDP security
descriptions). Both attribute capabilities are mandatory in this
configuration.
o The third configuration with configuration number 3 uses
transport capability 3 (RTP/AVPF) and mandatory attribute
capability 4 (the "rtcp-fb" attribute).
Bob receives the SDP offer from Alice. Bob supports Secure RTP,
Secure RTP with RTCP-based feedback and the SDP Capability
Negotiation extensions. Bob also supports SDP Security Descriptions,
but not MIKEY, and hence he generates the following answer:
v=0
o=- 24351 621814 IN IP4 192.0.2.2
s=
t=0 0
c=IN IP4 192.0.2.2
m=audio 54568 RTP/SAVP 98
a=rtpmap:98 AMR/8000
a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:WSJ+PSdFcGdUJShpX1ZjNzB4d1BINUAvLEw6UzF3|2^20|1:32
a=acfg:1 t=2 a=2
m=video 55468 RTP/SAVPF 31
a=rtpmap:31 H261/90000
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:AwWpVLFJhQX1cfHJSojd0RmdmcmVCspeEc3QGZiN|2^20|1:32
a=rtcp-fb:* nack
a=acfg:1 t=1 a=3,4
For the audio stream, Bob accepted the use of secure RTP, and hence
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the profile in the "m=" line is "RTP/SAVP". Bob also includes a
"crypto" attribute with his own keying material, and an "acfg"
attribute identifying actual configuration 1 for the audio media
stream from the offer, using transport capability 2 (RTP/SAVP) and
attribute capability 2 (the crypto attribute from the offer). For
the video stream, Bob accepted the use of secure RTP with RTCP-based
feedback, and hence the profile in the "m=" line is "RTP/SAVPF". Bob
also includes a "crypto" attribute with his own keying material, and
an "acfg" attribute identifying actual configuration 1 for the video
stream from the offer, using transport capability 1 (RTP/SAVPF) and
attribute capabilities 3 (the crypto attribute from the offer) and 4
(the "rtcp-fb" attribute from the offer).
When Alice receives Bob's answer, session negotiation has completed,
however Alice nevertheless chooses to generate a new offer using the
actual configuration. This is done purely to assist any
intermediaries that may reside between Alice and Bob but do not
support the capability negotiation extensions (and hence may not
understand the negotiation that just took place):
Alice's updated offer includes only SRTP for the audio stream SRTP
with RTCP-based feedback for the video stream, and it is not using
the SDP Capability Negotiation framework (Alice could have included
the capabilities as well is she wanted to):
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
t=0 0
c=IN IP4 192.0.2.1
m=audio 59000 RTP/SAVP 98
a=rtpmap:98 AMR/8000
a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
m=video 52000 RTP/SAVPF 31
a=rtpmap:31 H261/90000
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
a=rtcp-fb:* nack
The "m=" line for the audio stream now indicates that Alice is
offering to use secure RTP with PCMU or G.729, whereas the "m=" line
for the video stream indicates that Alice is offering to use secure
RTP with RTCP-based feedback and H.261. Each media stream includes a
"crypto" attribute, which provides the SRTP keying material, with
the same value again.
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Bob receives the SDP offer from Alice, which he accepts, and then
generates an answer to Alice:
v=0
o=- 24351 621814 IN IP4 192.0.2.2
s=
t=0 0
c=IN IP4 192.0.2.2
m=audio 54568 RTP/SAVP 98
a=rtpmap:98 AMR/8000
a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:WSJ+PSdFcGdUJShpX1ZjNzB4d1BINUAvLEw6UzF3|2^20|1:32
m=video 55468 RTP/SAVPF 31
a=rtpmap:31 H261/90000
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:AwWpVLFJhQX1cfHJSojd0RmdmcmVCspeEc3QGZiN|2^20|1:32
a=rtcp-fb:* nack
Bob includes the same crypto attribute as before, and the session
proceeds without change. Although Bob did not include any
capabilities in his answer, he could have done so if he wanted to.
Note that in this particular example, the answerer supported the
capability extensions defined here, however had he not, the answerer
would simply have ignored the new attributes received in step 1 and
accepted the offer to use normal RTP. In that case, the following
answer would have been generated in step 2 instead:
v=0
o=- 24351 621814 IN IP4 192.0.2.2
s=
t=0 0
c=IN IP4 192.0.2.2
m=audio 54568 RTP/AVP 98
a=rtpmap:98 AMR/8000
m=video 55468 RTP/AVP 31
a=rtpmap:31 H261/90000
a=rtcp-fb:* nack
Finally, if Bob had chosen to use session-level MIKEY instead of SDP
security descriptions instead, the following answer would have been
generated:
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
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t=0 0
c=IN IP4 192.0.2.1
a=key-mgmt:mikey AQEFgM0XflABAAAAAAAAAAAAAAYAyO...
m=audio 59000 RTP/AVP 98
a=rtpmap:98 AMR/8000
a=acfg:1 t=2 a=1
m=video 52000 RTP/SAVPF 31
a=rtpmap:31 H261/90000
a=rtcp-fb:* nack
a=acfg:1 t=1 a=1,4
It should be noted, that although Bob could have chosen session-
level MIKEY for one media stream, and SDP Security Descriptions for
another media stream, there are no well-defined offerer processing
rules of the resulting answer for this, and hence the offerer may
incorrectly assume use of MIKEY for both streams. To avoid this, if
the answerer chooses session-level MIKEY, then all secure RTP based
media streams SHOULD use MIKEY (this applies irrespective of whether
SDP Capability Negotiation is being used or not). Use of media-level
MIKEY does not have a similar constraint.
4.3. SRTP with Session-Level MIKEY and Media Level Security Descriptions as Alternatives
The following example illustrates how to use the SDP Capability
Negotiation framework to negotiate use of either MIKEY or SDP
Security Descriptions, when one of them is included as part of the
actual configuration, and the other one is being selected. The
offerer (Alice) wants to establish an audio and video session. Alice
prefers to use session-level MIKEY as the key management protocol,
but supports SDP security descriptions as well.
The example is illustrated by the offer/answer exchange below, where
Alice sends an offer to Bob:
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Security Descriptions and MIKEY. Since the potential configuration
is more preferred than the actual configuration, Bob (conceptually)
generates an internal potential configuration SDP that contains the
crypto attributes for the audio and video stream, but not the key-
mgmt attribute for MIKEY, thereby avoiding any ambiguity between the
two keying mechanisms. As a result, he generates the following
answer:
v=0
o=- 24351 621814 IN IP4 192.0.2.2
s=
t=0 0
c=IN IP4 192.0.2.2
m=audio 54568 RTP/SAVP 98
a=rtpmap:98 AMR/8000
a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:WSJ+PSdFcGdUJShpX1ZjNzB4d1BINUAvLEw6UzF3|2^20|1:32
a=acfg:1 a=-s:1
m=video 55468 RTP/SAVP 31
a=rtpmap:31 H261/90000
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:AwWpVLFJhQX1cfHJSojd0RmdmcmVCspeEc3QGZiN|2^20|1:32
a=acfg:1 a=-s:2
For the audio stream, Bob accepted the use of secure RTP using SDP
security descriptions. Bob therefore includes a "crypto" attribute
with his own keying material, and an "acfg" attribute identifying
actual configuration 1 for the audio media stream from the offer,
with the delete-attributes ("-s") and attribute capability 1 (the
crypto attribute from the offer). For the video stream, Bob also
accepted the use of secure RTP using SDP security descriptions. Bob
therefore includes a "crypto" attribute with his own keying
material, and an "acfg" attribute identifying actual configuration 1
for the video stream from the offer, with the delete-attributes ("-
s") and attribute capability 2.
Below, we illustrate the offer SDP, when Bob instead offers the
"crypto" attribute as the actual configuration keying mechanism and
"key-mgmt" as the potential configuration:
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
t=0 0
c=IN IP4 192.0.2.1
a=acap:1 key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
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m=audio 59000 RTP/SAVP 98
a=rtpmap:98 AMR/8000
a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=acap:2 rtpmap:98 AMR/8000
a=pcfg:1 a=-m:1,2
m=video 52000 RTP/SAVP 31
a=rtpmap:31 H261/90000
a=acap:3 crypto:1 AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
a=acap:4 rtpmap:31 H261/90000
a=pcfg:1 a=-m:1,4
Note how we this time need to perform delete-attributes at the
media-level instead of the session-level. When doing that, all
attributes from the actual configuration SDP, including the rtpmaps
provided, are removed. Consequently, we had to include these rtpmaps
as capabilities as well, and then include them in the potential
configuration, thereby effectively recreating the original rtpmap
attributes in the resulting potential configuration SDP.
5. Security Considerations
The SDP Capability Negotiation Framework is defined to be used
within the context of the offer/answer model, and hence all the
offer/answer security considerations apply here as well. Similarly,
the Session Initiation Protocol (SIP) uses SDP and the offer/answer
model, and hence, when used in that context, the SIP security
considerations apply as well.
However, SDP Capability Negotiation introduces additional security
issues. Its use as a mechanism to enable alternative transport
protocol negotiation (secure and non-secure) as well as its ability
to negotiate use of more or less secure keying methods and material
warrant further security considerations. Also, the (continued)
support for receiving media before answer combined with negotiation
of alternative transport protocols (secure and non-secure) warrant
further security considerations. We discuss these issues below.
The SDP Capability Negotiation framework allows for an offered media
stream to both indicate and support various levels of security for
that media stream. Different levels of security can for example be
negotiated by use of alternative attribute capabilities each
indicating more or less secure keying methods as well as more or
less strong ciphers. Since the offerer indicates support for each of
these alternatives, he will presumably accept the answerer seemingly
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selecting any of the offered alternatives. If an attacker can modify
the SDP offer, he can thereby force the negotiation of the weakest
security mechanism that the offerer is willing to accept. This may
enable the attacker to compromise the security of the negotiated
media stream. Similarly, if the offerer wishes to negotiate use of a
secure media stream (e.g. secure RTP), but includes a non-secure
media stream (e.g. plain RTP) as a valid (but less preferred)
alternative, then an attacker that can modify the offered SDP will
be able to force the establishment of an insecure media stream. The
solution to both of these problems involves the use of integrity
protection over the SDP. Ideally, this integrity protection provides
end-to-end integrity protection in order to protect from any man-in-
the-middle attack; secure multiparts such as S/MIME [RFC3851]
provide one such solution, however S/MIME requires use and
availability of a Public Key Infrastructure (PKI). A slightly less
secure alternative when using SIP, but generally much easier to
deploy in practice (since it does not require a PKI), is to use SIP
Identity [RFC4474]; this requires the existence of an authentication
service (see [RFC4474]). Yet another, and considerably less secure,
alternative is to use hop-by-hop security only, e.g. TLS or IPSec
thereby ensuring the integrity of the offered SDP on a hop-by-hop
basis. Note however that SIP proxies or other intermediaries
processing the SIP request at each hop are able to perform a man-in-
the-middle attack by modifying the offered SDP.
Per the normal offer/answer procedures, as soon as the offerer has
generated an offer, the offerer must be prepared to receive media in
accordance with that offer. The SDP Capability Negotiation preserves
that behavior for the actual configuration in the offer, however the
offerer has no way of knowing which configuration (actual or
potential) configuration was selected by the offerer, until an
answer indication is received. This opens up a new security issue
where an attacker may be able to interject media towards the offerer
until the answer is received. For example, the offerer may use plain
RTP as the actual configuration and secure RTP as an alternative
potential configuration. Even though the answerer selects secure
RTP, the offerer will not know that until he receives the answer,
and hence an attacker will be able to send media to the offerer
meanwhile. The easiest protection against such an attack is to not
offer use of the non-secure media stream in the actual
configuration, however that may in itself have undesirable side-
effects: If the answerer does not support the secure media stream
and also does not support the capability negotiation framework, then
negotiation of the media stream will fail. Alternatively, SDP
security preconditions [RFC5027] can be used. This will ensure that
media is not flowing until session negotiation has completed and
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hence the selected configuration is known. Use of preconditions
however requires both sides to support them. If they don't, and use
of them is required, the session will fail. As a (limited) work
around to this, it is RECOMMENDED that SIP entities generate an
answer SDP and send it to the offerer as soon as possible, for
example in a 183 Session Progress message. This will limit the time
during which an attacker can send media to the offerer. Section 3.9.
presents other alternatives as well.
Additional security considerations apply to the answer SDP as well.
The actual configuration attribute tells the offerer which potential
configuration the answer was based on, and hence an attacker that
can either modify or remove the actual configuration attribute in
the answer can cause session failure as well as extend the time
window during which the offerer will accept incoming media that does
not conform to the actual answer. The solutions to this SDP answer
integrity problem are the same as for the offer, i.e. use of end-to-
end integrity protection, SIP identity, or hop-by-hop protection.
The mechanism to use depends on the mechanisms supported by the
offerer as well as the acceptable security trade-offs.
As described in Section 3.1. , SDP Capability Negotiation
conceptually allows an offerer to include many different offers in a
single SDP. This can cause the answerer to process a large number of
alternative potential offers, which can consume significant memory
and CPU resources. An attacker can use this amplification feature to
launch a denial of service attack against the answerer. The answerer
MUST protect itself from such attacks. As explained in Section 3.10.
, the answerer can help reduce the effects of such an attack by
first discarding all potential configurations that contain
unsupported transport protocols, unsupported or invalid mandatory
attribute capabilities, or unsupported mandatory extension
configurations. The answerer SHOULD also look out for potential
configurations that are designed to pass the above test, but
nevertheless produce a large number of potential configuration SDPs
that cannot be supported.
A possible way of achieving that is for an attacker to find a
valid session-level attribute that causes conflicts or otherwise
interferes with individual media description configurations. At
time of publication of this document, we do not know of such an
SDP attribute, however this does not mean it does not exist, or
that it will not exist in the future. If such attributes are found
to exist, implementers should explicitly protect against them.
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A significant number of valid and supported potential configurations
may remain. However, since all of those contain only valid and
supported transport protocols and attributes, it is expected that
only a few of them will need to be processed on average. Still, the
answerer MUST ensure that it does not needlessly consume large
amounts of memory or CPU resources when processing those as well as
be prepared to handle the case where a large number of potential
configurations still need to be processed.
6. IANA Considerations6.1. New SDP Attributes
The IANA is hereby requested to register the following new SDP
attributes as follows:
Attribute name: csup
Long form name: Supported capability negotiation extensions
Type of attribute: Session-level and media-level
Subject to charset: No
Purpose: Option tags for supported SDP capability
negotiation extensions
Appropriate values: See Section 3.3.1. of RFCXXXX
-- Note to RFC editor:
-- replace RFCXXXX by this RFC number
Contact name: Flemming Andreasen, fandreas@cisco.com
Attribute name: creq
Long form name: Required capability negotiation extensions
Type of attribute: Session-level and media-level
Subject to charset: No
Purpose: Option tags for required SDP capability
negotiation extensions
Appropriate values: See Section 3.3.2. of RFCXXXX
-- Note to RFC editor:
-- replace RFCXXXX by this RFC number
Contact name: Flemming Andreasen, fandreas@cisco.com
Attribute name: acap
Long form name: Attribute capability
Type of attribute: Session-level and media-level
Subject to charset: No
Purpose: Attribute capability containing an attribute
name and associated value
Appropriate values: See Section 3.4.1. of RFCXXXX
-- Note to RFC editor:
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-- replace RFCXXXX by this RFC number
Contact name: Flemming Andreasen, fandreas@cisco.com
Attribute name: tcap
Long form name: Transport Protocol Capability
Type of attribute: Session-level and media-level
Subject to charset: No
Purpose: Transport protocol capability listing one or
more transport protocols
Appropriate values: See Section 3.4.2. of RFCXXXX
-- Note to RFC editor:
-- replace RFCXXXX by this RFC number
Contact name: Flemming Andreasen, fandreas@cisco.com
Attribute name: pcfg
Long form name: Potential Configuration
Type of attribute: Media-level
Subject to charset: No
Purpose: Potential configuration for SDP capability
negotiation
Appropriate values: See Section 3.5.1. of RFCXXXX
-- Note to RFC editor:
-- replace RFCXXXX by this RFC number
Contact name: Flemming Andreasen, fandreas@cisco.com
Attribute name: acfg
Long form name: Actual configuration
Type of attribute: Media-level
Subject to charset: No
Purpose: Actual configuration for SDP capability
negotiation
Appropriate values: See Section 3.5.2. of RFCXXXX
-- Note to RFC editor:
-- replace RFCXXXX by this RFC number
Contact name: Flemming Andreasen, fandreas@cisco.com
6.2. New SDP Capability Negotiation Option Tag Registry
The IANA is hereby requested to create a new SDP Capability
Negotiation Option Tag registry. An IANA SDP Capability Negotiation
option tag registration MUST be documented in an RFC in accordance
with the [RFC5226] Specification Required policy. The RFC MUST
provide the name of the option tag, a syntax and a semantic
specification of any new SDP attributes and any extensions to the
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potential and actual configuration attributes provided in this
document. New SDP attributes that are intended to be capabilities
for use by the capability negotiation framework MUST adhere to the
guidelines provided in Section 3.4.3. Extensions to the potential
and actual configuration attributes MUST adhere to the syntax
provided in Section 3.5.1. and 3.5.2.
The option tag "cap-v0" is defined in this document and the IANA is
hereby requested to register this option tag.
6.3. New SDP Capability Negotiation Potential Configuration Parameter Registry
The IANA is hereby requested to create a new SDP Capability
Negotiation Potential Configuration Parameter registry. An IANA SDP
Capability Negotiation potential configuration registration MUST be
documented in an RFC in accordance with the [RFC5226] Specification
Required policy. The RFC MUST define the syntax and semantics of
each new potential configuration parameter. The syntax MUST adhere
to the syntax provided for extensions in Section 3.5.1. and the
semantics MUST adhere to the semantics provided for extensions in
Section 3.5.1. and 3.5.2. Associated with each registration MUST be
the encoding name for the parameter as well as a short descriptive
name for it.
The potential configuration parameters "a" for "attribute" and "t"
for "transport protocol" are defined in this document and the IANA
is hereby requested to register these.
7. Acknowledgments
The SDP Capability Negotiation solution defined in this document
draws on the overall capability negotiation framework that was
defined by [SDPng]. Also, the SDP Capability Negotiation solution is
heavily influenced by the discussions and work done by the SDP
Capability Negotiation Design Team. The following people in
particular provided useful comments and suggestions to either the
document itself or the overall direction of the solution defined in
here: Francois Audet, John Elwell, Roni Even, Miguel Garcia, Robert
Gilman, Cullen Jennings, Jonathan Lennox, Matt Lepinski, Jean-
Francois Mule, Joerg Ott, Colin Perkins, Jonathan Rosenberg, Thomas
Stach, and Dan Wing.
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Internet-Draft SDP Capability Negotiation July 20088. Change Log8.1. draft-ietf-mmusic-sdp-capability-negotiation-09
Incorporated Working Group Chair review comments and a few additional
comments as follows:
o Clarified that the "a=creq" attribute MUST NOT be used in an
answer (Section 3.6.2. ).
o Various editorial changes throughout.
8.2. draft-ietf-mmusic-sdp-capability-negotiation-08
Incorporated Working Group Last Call comments as follows:
o Added second offer/answer exchange to introductory example, fixed
minor error in that example, and deleted similar example in the
Examples Section.
o Fixed "type=value" semantic error in the attribute capability
definition.
o Clarified that consecutive numbering of capabilities and
potential configurations is not required.
o Fixed inconsistency for which parameters to include in the "acfg"
attribute.
o Changed second offer/answer exchange from MAY to SHOULD strength.
o Clarified there should be a combined second offer/exchange when
using ICE.
o Moved RFC 3407 to informative references.
o Various editorial clarifications.
8.3. draft-ietf-mmusic-sdp-capability-negotiation-07
o Removed the ability to have attribute capabilities provide
attribute names without values, when those attributes otherwise
require an associated value.
o Document no longer obsoletes RFC 3407 but instead recommends that
it is being used instead of RFC 3407.
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o Added ability to specific that specific extensions in a potential
configuration are mandatory.
o Changed ABNF for extension-config-list in potential
configurations.
o Removed the redundant "a=" part of attribute capabilities.
o Clarified what it means to support an attribute capability in the
offer/answer procedures.
o Changed "a=acap" attribute and offer/answer procedures to include
only the known and supported attribute capabilities.
o Added new section on indicating bandwidth usage.
8.4. draft-ietf-mmusic-sdp-capability-negotiation-06
o Added additional background text on terminology used, and a new
section on the negotiation model.
o Allowed for session-level attribute capabilities to contain
media-level only attributes, albeit the base framework does not
define (or allow) them to be used in a potential configuration
(extensions may change that)
o Disallowing multiple "a=tcap" attributes at the session-level
and/or on a per media description basis; at most one at the
session-level and per media description now.
o Changed the "a=pcfg" attribute to make a potential configuration
list optional in order to allow for the actual configuration to
be referenced.
o Removed the ability to delete and replace individual attributes
from the actual configuration SDP.
o Introduced the notion of mandatory and optional attribute
capabilities in a potential configuration and updated the
"a=pcfg" attribute and associated procedures accordingly.
o Specified that mandatory attribute capabilities and the transport
protocol (if any) from a potential configuration need to be
supported in order to select that potential configuration.
Offer/answer procedures updated accordingly as well.
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o Noted potential interaction and synchronization issues with use
of session-level attributes and attribute capabilities and added
recommendation to avoid use of session-level attributes when
possible.
o Fixed error in "a=acfg" grammar (missing config-number) and
updated attribute definition in accordance with the "a=pcfg"
attribute changes.
o Updated text associated with processing media before answer to
allow for playing out garbage or discard until answer received.
Additional detail on alternative solutions provided as well.
o Added recommendation to send back answer SDP as soon as possible,
when a potential configuration different from the actual
configuration has been chosen.
o Added new section on interactions with SIP option tags.
o Added new section on dealing with large number of potential
configurations.
o Added new section on SDP capability negotiation and
intermediaries.
o Updated examples in accordance with other changes and to
illustrate use of mandatory and optional attribute capabilities
in a potential configuration.
o Updated security considerations to address potential denial of
service attack caused by large number of potential
configurations.
o Various editorial updates throughout.
8.5. draft-ietf-mmusic-sdp-capability-negotiation-05
o Allowed for '<type>=<value>' attributes to be listed as attribute
capabilities the attribute name only.
o Changed IP-address to conform to RFC 3330 guidelines.
o Added section on relationship to RFC 3407 and "Obsoletes: 3407"
in the front.
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o Disallowed use of white space in a number of places for more
consistency with existing SDP practice
o Changed "csup" and "creq" attributes to not allow multiple
instances at the session-level and multiple instances per media
description (only one for each now)
o Changed to not require use of "creq" with base option tag ("cap-
v0").
o Relaxed restrictions on extension capabilities
o Updated potential configuration attribute syntax and semantics.
In particular, potential configuration attributes can now replace
and delete various existing attributes in original SDP to better
control potential attribute interactions with the actual
configuration while preserving message size efficiency.
o Updated actual configuration attribute to align with the updates
to the potential configuration attributes.
o Updated offer/answer procedures to align with other changes.
o Changed recommendation for second offer/answer exchange to "MAY"
strength, unless for the cases where it is known or suspected
that it is needed.
o Updated ICE interactions to explain how the new attribute
delete/replace features can solve certain potential interactions.
o Updated rtpmap and fmtp section to allow potential configurations
to use remapped payload types in attribute capabilities for
rtpmaps and fmtp parameters.
o Added section on direction attributes.
o Added another example showing SRTP with session-level MIKEY and
SDP Security Descriptions using the attribute capability DELETE
operator.
8.6. draft-ietf-mmusic-sdp-capability-negotiation-04
The following are the major changes compared to version -03:
o Added explicit ordering rules for attributes added by potential
configurations.
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Internet-Draft SDP Capability Negotiation July 2008
o Noted that ICE interaction issues (ice-tcp specifically) may not
be as clear as originally thought.
o Added considerations on using rtpmap and fmtp attributes as
attribute capabilities.
o Added multiple transport protocol example.
o Added session-level MIKEY and media level security descriptions
example.
8.7. draft-ietf-mmusic-sdp-capability-negotiation-03
The following are the major changes compared to version -02:
o Base option tag name changed from "v0" to "cap-v0".
o Added new section on extension capability attributes
o Firmed up offer/answer procedures.
o Added security considerations
o Added IANA considerations
8.8. draft-ietf-mmusic-sdp-capability-negotiation-02
The following are the major changes compared to version -01:
o Potential configurations are no longer allowed at the session
level
o Renamed capability attributes ("capar" to "acap" and "ctrpr" to
"tcap")
o Changed name and semantics of the initial number (now called
configuration number) in potential configuration attributes; must
now be unique and can be used as a handle
o Actual configuration attribute now includes configuration number
from the selected potential configuration attribute
o Added ABNF throughout
o Specified that answerer should include "a=csup" in case of
unsupported required extensions in offer.
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Internet-Draft SDP Capability Negotiation July 2008
o Specified use of second offer/answer exchange when answerer
selected a potential configuration
o Updated rules (and added restrictions) for referencing media- and
session-level capabilities in potential configurations (at the
media level)
o Added initial section on ICE interactions
o Added initial section on receiving media before answer
8.9. draft-ietf-mmusic-sdp-capability-negotiation-01
The following are the major changes compared to version -00:
o Media capabilities are no longer considered a core capability and
hence have been removed. This leaves transport protocols and
attributes as the only capabilities defined by the core.
o Version attribute has been removed and an option tag to indicate
the actual version has been defined instead.
o Clarified rules for session-level and media level attributes
provided at either level as well how they can be used in
potential configurations.
o Potential configuration parameters no longer have implicit
ordering; an explicit preference indicator is now included.
o The parameter name for transport protocols in the potential and
actual configuration attributes have been changed "p" to "t".
o Clarified operator precedence within potential and actual
configuration attributes.
o Potential configurations at the session level now limited to
indicate latent capability configurations. Consequently, an
actual configuration attribute can no longer be provided at the
session level.
o Cleaned up capability and potential configuration terminology -
they are now two clearly different things.
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Version 00 is the initial version. The solution provided in this
initial version is based on an earlier (individual submission)
version of [SDPCapNeg]. The following are the major changes compared
to that document:
o Solution no longer based on RFC 3407, but defines a set of
similar attributes (with some differences).
o Various minor changes to the previously defined attributes.
o Multiple transport capabilities can be included in a single
"tcap" attribute
o A version attribute is now included.
o Extensions to the framework are formally supported.
o Option tags and the ability to list supported and required
extensions are supported.
o A best-effort SRTP example use case has been added.
o Some terminology change throughout to more clearly indicate what
constitutes capabilities and what constitutes configurations.
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[BESRTP] Kaplan, H., and F. Audet, "Session Description Protocol
(SDP) Offer/Answer Negotiation for Best-Effort Secure
Real-Time Transport Protocol, Work in progress, August
2006.
[ICE] J. Rosenberg, "Interactive Connectivity Establishment
(ICE): A Methodology for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", work in progress,
September 2007.
[ICETCP] J. Rosenberg, "TCP Candidates with Interactive
Connectivity Establishment (ICE)", work in progress, July
2007.
[SAVPF] Ott, J., and E Carrara, "Extended Secure RTP Profile for
RTCP-based Feedback (RTP/SAVPF)", Work in Progress, May
2007.
[SDPCapNeg] Andreasen, F. "SDP Capability Negotiation", work in
progress, December 2006.
[SDPng] Kutscher, D., Ott, J., and C. Bormann, "Session
Description and Capability Negotiation", Work in Progress,
February 2005.
Author's Addresses
Flemming Andreasen
Cisco Systems
Edison, NJ
Email: fandreas@cisco.com
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